Hello fellow Autopians, and welcome to another edition of Ask An Engineer. This time, we are going to answer a question sent to me by Thomas who asked me why the 2023 Honda Accord Hybrid EX-L gets an estimated 51 mpg in the city while the other Hybrids (Sport, Sport-L, and Touring) only get 46 mpg. Looking at the specs for each of these vehicles, the only difference appears to be the tires, so Thomas’ question got me thinking about the influence tires have on vehicle efficiency both for ICE cars and EVs. It also reminded me of a conversation I had years ago with a friend who owned a second gen Prius. He had put summer tires and new wheels on it a few months before because he liked the look but he noticed a definite drop in fuel economy. He didn’t have any numbers to share but he said it was very noticeable. The key to understanding what’s going on here is the concept of parasitic losses.
Parasitic Losses
Whenever energy gets converted from one form to another, such as from gasoline to twisting of an engine’s crankshaft, or from electricity to the rotation of the motor’s rotor, some of the energy will always be lost in the form of heat. There is no perfect energy conversion in this world so some of it will always end up as waste. In the case of a typical ICE car, about 66% of the energy contained in each gallon of gasoline gets lost to heat and only 33% ends up creating motion for the car. EVs, on the other hand, are much more efficient, with over 90% of the electrical energy going into motion and less than 10% getting lost to heat.
Unfortunately, that’s not where the energy losses stop. Between the motor and the road, there are more mechanisms such as transmissions, bearings, seals and lubricants, and each of these has a certain amount of friction which eats away at the energy being produced by the motor. Modern bearings and seals have very low friction, but it’s not zero. Add up all the different bearings and seals in the drivetrain and they result in a pretty significant amount of friction, and as we all know from rubbing our hands together, friction produces heat. All this heat represents energy that is dissipated to the air. Of course, this energy has to come from somewhere and the only source of energy in a car is whatever is in the tank, be it gasoline, diesel, or electricity.
All these little energy losses are like little parasites living off the lifeblood of your car, like a tick living of the blood of a dog, hence the term “parasitic losses”.
One of the largest energy losses comes by way of the brakes. We can’t really call the brakes parasitic losses because we actually get some useful value out of them, but they still cause energy to be lost. When you hit the brakes, the energy of motion is converted into heat which is then dissipated into the air. This is of course the sole function of the brakes: convert motion energy (kinetic energy) into heat energy. And since the energy of motion is now reduced, the speed of the vehicle is reduced. But what if we could convert the energy of motion into some other form of energy that we could reuse later? This is what a hybrid vehicle is all about. It’s also what regen in an EV does. In both cases, the electric motor that is part of the drivetrain is turned into a generator, and instead of using brakes to turn motion into heat, they turn motion into electricity to be stored in the battery for later use. This is a fundamental concept behind how hybrids work ,and key to why they get so much better fuel economy.
Having said all that, and as interesting as all that may be, how does any of this relate to Thomas’ question? The vehicles whose fuel economy figures he’s comparing are hybrids, so that can’t be what’s going on here. As I mentioned earlier, the only difference between the vehicles in the Honda lineup appears to be the tires, and this gets to the biggest source of parasitic losses in a car: the tires.
As a tire rolls down the road, the deformation that occurs at the contact patch (temporarily transforming the round shape of the tire into a flat area against the road) and the movement of the rubber-blocks against the road surface absorb some of the energy that is moving the car forward. This energy gets converted into heat, which is the reason tires are warmer after you drive on them for a while. This concept is called the rolling resistance (RR) of the tire.
Tire Rolling Resistance
All tires have some RR, but the amount varies widely, and is a function of (among other things) the chemistry of the rubber used in the tread, the construction of the belts and cords, and the tire size. Summer tires tend to have higher RR due mainly to the chemistry of the rubber that gives them such high grip. All-season tires tend to have lower RR, but then they also have lower grip. Wider tires tend to have higher RR while a larger outer-diameter (not wheel diameter) tends to reduce it. This is why you see large narrow tires on cars such as the BMW i3.
Since tire manufacturers started putting silica in tire rubber, RR has been markedly reduced without compromising other things tires need to do, like provide grip in dry and wet conditions, and absorb impacts from the road. Modern tires use the latest technologies in construction and rubber chemistry to reduce their energy consumption while still maintaining good grip. In the past, low RR was inevitably accompanied by poor traction, especially on wet roads. These newer tires still have some similar compromises, but they are getting much better, and I’m sure we will see many more developments in the future that make them even better.
Let’s look at RR in more detail. This diagram shows a tire with a flat portion on the bottom where it meets the road. This is the contact patch. It’s quite exaggerated here but that’s just to illustrate the point.
Imagine a piece of the tread as the tire is rolling down the road. At first, the piece of tread is round, but as it comes around and meets the road, this piece of tread has to change shape from round to flat. Any time we ask a material to change shape, it takes energy. The same happens here’ we have to put energy into the tire to make the tread change shape. When our piece of tread reaches the end of the contact patch and returns to a round shape, we will get some of that energy back, but not all. The bit of energy that is lost turns into heat and warms up the tire.
The other thing that is going on is when the tread is asked to change from round to flat, is that materials of different lengths are now being asked to be the same length. Let’s look at the angle between the start and end of the contact patch. It’s about 60 degrees:
Let’s look at a 60 degree section of an undeformed portion of the tire. Notice also that the belts (shown in yellow here) and the tread are not the same distance from the center of the tire. If we assume our tire is a P235/50R18, then the distances might be something like this:
The length of belt (Lb) and the length of tread rubber (Lt) contained in our 60 degree portion of the tire are not the same length, since the tread is farther outboard. In this case, the length of belt is about 13.61″ while the length of rubber is about 14.27″. That’s a difference of over 5/8″. Now, when this same 60-degree portion of the tire rolls around to become the contact patch, both the 13.61″ length of belt and the 14’27” length of rubber need to fit into the same straight length of contact patch. Just looking at the bottom of the picture above, you see that the tread and belt are parallel, and thus the same length since they’re no longer arcs at different distances from a center point — they’re lines.
And since the belts are made of steel and Kevlar and other strong materials, they do not want to change significantly in length, so the rubber becomes forced to conform to the smaller 13.61″ length of the belts and gets squeezed in the process. This squeezing doesn’t happen all at once, but slowly progresses as the tire rolls over the contact patch. The result is a lot of squirming of the rubber between the belts and the road surface, causing more heat to build up in the rubber. If there is a lot of space between the tread blocks, like in our diagram or like there would be in an all-season tire, the rubber has somewhere to go as it is getting squeezed, but if the tread blocks are close together (i.e. so there’s more material touching the ground) like in a summer tire, the rubber has less room to move around. This is part of the reason summer tires tend to have higher RR than all-season or winter tires.
RR is designated by a unitless number that represents the force required to roll the tire forward for every 1000 units of force the tire is carrying. For example, if a tire is carrying 1000 lbs of weight, and it takes 10 lbs of force to roll this tire forward, then its RR is said to be 10, or 10 lbs/1000lbs. In the metric system this tire would have a RR of 10 kg/ton. Most everyday tires these days have RR numbers between about 8 and 12, with high performance summer tires at the high end of that range and all-season tires at the low end. There are tires that are lower than that, and they are becoming more and more common especially now that EV’s with their high range requirements are becoming more popular.
Of course, all of this is an over-simplification of what’s really going on inside your tires, but you can see how they would have an impact on fuel economy and EV range.
In fact, according to Seong-su Kim, a senior researcher at Hyundai Motor Group, reducing tire RR by 1.o leads to a range increase of about 5%. That’s a big deal; on a car with 250 miles of range, that’s over 12 miles. As a result, when Hyundai was developing its new E-GMP platform, to ensure adequate range, the team’s target was a rolling resistance no higher than 6.5.
While an RR of 6.5 is considered to be very low, it is certainly possible with the latest tire technology. It just depends on what you are willing to compromise to get it.
For more on the impacts of rolling resistance and tire size, Engineering Explained does a good job (see above) going into the details although he ends up concluding that wheel diameter has an impact on range based on various Tesla range specs. The reality is that it all boils down to tire RR. If we had a 19″ tire and a 21″ tire with the same RR, the range impact would be zero.
The Honda Tires
So, how does this all relate to the Honda Accord Hybrid? Since tires are some of the biggest energy hogs in your car, changing the RR of your tires can have a dramatic effect on your fuel economy or EV range.
According to Tirerack.com, the OEM tire for the Honda Accord Hybrid EX-L is either a Hankook Kinergy GT or a Michelin Energy Saver A/S, both of which are P225/50R17, while the tires for the Sport, Sport-L, and Touring is either a Michelin Primacy MXM4 all-season or a Goodyear Eagle Touring all-season in a P235/40R19 size. The Primacy MXM4 is the same tire we used at Tesla on the Model S and at that time, if my memory serves, the RR was about 8.5.
Let’s suppose the rolling resistance of the EX-L tires had the same target as Hyundai used for their E-GMP platform. This mean a RR of 6.5, which is 2.0 lower than the Primacy MXM4 tires on the other cars. Based on Hyundai’s numbers, this would translate into about a 10% improvement in fuel economy, which is exactly the difference between the EX-L and the other Honda hybrids EPA ratings.
For a modern ultra low rolling resistance tire, 6.5 is not an unrealistic number, so it is entirely possible that the improvement in the fuel economy of the Honda Accord Hybrid EX-L is solely due to the difference in tires the car is equipped with. It really does make that much of a difference. Of course, to get such a low RR, it is possible that Honda had to make some compromises in performance, which they felt were unacceptable on the higher end hybrids. This may explain why they didn’t put the same tires on all variants.
The difference in range due to tires is evident at Tesla as well. The base Model S with 19″ tires has an EPA range of 405 miles while the range with the larger 21″ wheels is 375 miles. The difference is that the 19″ tires are all-season low RR tires while the 21″ers are high performance summer tires with more rolling resistance.
Another example of the impact tires have on fuel economy and EV range is the difference between the range of the Hyundai Ioniq 6 SE and SEL. The RWD SE comes with a P225/55R18 and has a 361 mile EPA range while the RWD SEL comes with P245/40R20 tires and has an EPA range of 305 miles. According to Hyundai’s own website, everything else that would impact range appears to be the same for both versions: weight is the same (4,222-4,376 lbs), battery size is the same (77.4 kWh), and both use the same 168 kW/350 Nm motors. The only difference appears to be the tires, so here again we see how just changing tires can have a significant impact on EV range, just like it does on ICE fuel economy.
If you have any other car engineering questions, please send them to AskAnEngineer@TheAutopian.com and I will do my best to answer them. As my college professors used to say: “There is no such thing as a dumb question, only dumb answers”.
As a wise man once said…
“The tires are the thing on your car that make contact with the road”
Trey Anastasio!
Everything your car does happens through the tires!
Bummed is what you are when you go out to your car and it’s been towed.
The wheels are a critical part of the vehicle that transmits all the power all the time between the car itself and the ground. These forces tend to be large, just the weight of the vehicle is several thousands pounds so even small changes there tend to have noticeable effects, and sometimes dramatic effects.
Despite this tires are often overlooked by the consumer. By choosing an “eco” tire over a “regular” tire, the driver can save a meaningful amount of money over the years in fuel, and equally, performance tires are needed for a high performance car perform the way it’s designers intended.
The difference between wheel and tire sizes is really weird. In general I prefer the tallest and the skinniest tires I can get (for cars) but for bicycles taller wheels are easier to damage than shorter wheels. It’s a serious conundrum.
Why skinniest? I generally buy the tallest and widest tire that will fit.
Well they’re easier to avoid road obstacles with much like how a dirtbike can avoid a ton of road hazards instead of being forced to deal with them, and on pavement they tend to cut through the snow down to the hard pavement beneath where wider tires float on top of it. Also provided two tires are the same overall height on the same automobile the skinnier tires are generally lighter and they allow the automobile to have a better turning circle.
If you look at the first mass production cars ever made they have extremely tall and very skinny tires and wheels. As more roads were paved you saw tire and wheel diameters decrease. Look up Ford Model T articulation on google images, it’ll put any other mass production automobile to shame.
hmm.. not sure about your claim about narrow tires decreasing turning radius, all my lifetime empirical data suggests that wider tires help with the turn radius
How would wider tires IMPROVE turn radius? Shorter tires perhaps but WIDER?
Thanks very much for your excellent explanation of an important topic Huibert. As I get older, I’m much less obsessed with +1, +2, etc… wheel ‘upgrades’ than I was as a teen. In fact, I now think some cars look better without bigger wheels… particularly cars with some degree of retro-inspired styling. Sure, a Mini Cooper looks meaner with giant wheels stuffed under it, but frankly, the styling of the car is such that taller sidewalls (with the original, smaller wheel size) just look “right.”
That’s a shockingly huge range difference on the Hyundai Ionic 6 SE vs SEL… sticking with the base/regular tire on the SE seems like a no-brainer, given that it’s likely to get 56 miles of extra range.
Multiple thumbs-up. On big rims they look like a small child wearing their parent’s shoes. Not to mention the crap ride-quality of BMW-specced run-flat tyres, exponentially worse for each 1″ increase in rim diameter/1/2″ reduction in sidewall height.
Some of us think all cars look better without bigger wheels……
Agree 100%. We bought a new vehicle last October, and the dealer kept asking us- “Are you sure you want the 18” wheels? The 19” and 20” are very popular!”I kept responding- “Well if there are 17” wheels available, I’ll take those.”
The SUV just looks better with a little less wheel and a little more rubber. Rides way better too.
True that. My shitbox is a Bugeye wrx that came stock with 16” wheels. Everyone put 17 & 18” wheels on them which I don’t care for as I’m not a track rat: I play on dirt & gravel, and want sidewalls to be tall. I’ll admit that some 17s look good on a lowered one-but I’m raising mine for clearance.
and (each to their own, tho), I really dislike 18-20” wheels on old iron that came with 14 or 15” wheels. Saw a GM square body truck this morning with massive wheels & seemingly 1” sidewalls. The truck’s body was very nice, but those wheels ruined the overall look for me. A shame
Thank You Mr. Mees for another informative post. I would like your take on low profile tires. My current steed is shod with 225/45R17 front 235/40R17 rear, and I wouldn’t want any lower. Several of my neighbors have had to replace tires and wheels after pothole encounters, and these were common Honda and Audi sedans, not track cars.
When I was at Ford, the rule of thumb was that the width of the tire multiplied by the aspect ratio had to be greater than 92.5. Anything less would lead to inevitable pinch flats after a pothole strike. In your case, 225*0.45=101.25 and 235*0.4=94. In theory you should be ok but I wouldn’t go lower on those rear tires. It all depends on where you live. I live in northern California where our roads are actually much worse than you would imagine and my 2015 Mustang has sidewalls that are exactly 92.5 mm tall and I have never had an issue with pinch flats. Now, if I were living in a snow climate where potholes are worse, I might have a problem.
Thanks for the reply. I miss spoke, my rears are 245/40R17. My Real Question is why are manufactures so enamored with ultra low profiled tires as stock on cars and SUVs even that have no intention of lateral G testing. The drawbacks ( harsher ride, more likely damaged ) far outweigh the benefit of less sidewall flex, crisper handling.
*This*
Thank you Huibert for having the talk. It’s long overdue.
The bit about tread pattern really cleared something up for me. I totally expected to see a fuel economy drop when I switch between summer and winter tires, yet have not really observed one. It might be my modest (by today’s standard) 16″ rim size. With all else being equal like tire profile, my winter and all seasons, get me about the same efficiency, even with the assumption that my winters are probably heavier and on steelies. Of course, cold weather driving has other impacts on efficiency, but I really thought the different tires would matter more too.
I understand why larger rims on sports cars provide more clearance for bigger brakes, but I’m confounded by why they are the rage on EVs (fashion?). If the braking energy is absorbed in regen, there should be no need for bigger disks.
I believe that EVs still need bigger brakes because they are heavier than ICE cars. Regen does a lot, but it is not always the same. If the battery is 100% full, for instance, the brakes will do most of the stopping because there is no place for that energy to go.
So they do need larger wheels. Although yes, most of the larger wheel sizes within the same model are just for looks…
Inboard brakes might be a good solution for EVs and allow for smaller wheels. I’m assuming of course the inboard brakes would be in line with the motors & therefore have minimal impact to space & packaging constraints, especially related to the HV battery pack
Inboard brakes are a package nightmare. There is never enough space close to the motor and the diameter of the brake rotor would mean it sticks down below the bottom of the car. Also, there is not nearly enough airflow in that area to keep them cool.
Regen-braking is only going on during relatively minor braking. If you brake hard, the traditional brakes are used.
Some of the other commenters are right. EV’s still need properly sized brakes since you can’t count on regen always being available. The reason why you might not be seeing an improvement in fuel economy going from winter to summer tires is that true winter tires have their own issues with rolling resistance. They have a lot of hysteresis in the rubber compounds to give them traction on snow but this hysteresis also absorbs energy.
“The reality is that it all boils down to tire RR. If we had a 19″ tire and a 21″ tire with the same RR, the range impact would be zero.”
This is simply not true. The wheel diameter affects aero, and thus range, profoundly.
I have personally tested this on a Nissan LEAF. Take a LEAF with 16 inch wheels and put Bridgestone Ecopia tires on it. Now swap to the factory 17 inch wheels and put the same makel/model tire on it: you’ll get about 10-15% less range on the larger rims even though the tire is virtually the same.
Bigger rims stir up a lot more air and create a lot more drag. They also tend to have larger outside tire diameters which raise the car slightly which further hurts aero and increases drag.
Yeah, I was also very confused by this. I assume on paper, theoretically yes. But in the real world, no.
When the first Tesla Model S came out, the standard 19′ rims looked absurdly inefficient. They were replaced with the first refresh of the car, while the larger, more aerodynamic 21′ wheels remained.
https://cdn05.carsforsale.com/00f58fbcb35ffddd2a91c85a87f0b496af/1280×960/2015-tesla-model-s-85d-awd-4dr-liftback.jpg
https://media.wired.com/photos/59337e9b68cb3b3dc4099359/master/w_2560%2Cc_limit/09132012-TESLA-MODEL-S-094edit.jpg
(The 19″ wheels did not disappear, of course. They were replaced by a different design that kind of mimicked the existing 21″)
Agreed. Some Tesla models come with ‘aero’ wheel covers that reduce the drag from wheel surfaces. Car and Driver tested these, and found that removing them (with the identical wheel and tire combination) was a 3.2% difference in predicted range at 50 MPH: https://www.caranddriver.com/news/a30169467/tesla-model-3s-aero-wheel-covers-efficiency-test/
It makes sense that wheel size and design would also have an impact (keeping the tire design and size constant).
I’d like to see some narrower tires with real sidewalls! Every car doesn’t have to have dubs yo
Cars with 20-22″ wheels when 15-16 is just fine.
I agree but the suspension and brakes still need to fit inside the wheels. In the days of 15″ wheels, the brakes were hopelessly small. Most modern brakes need at least a 17″ wheel to fit into.
Hey now, my Miata is on 15s and stops just fine! Of course, it also weighs less than half as much as the average vehicle on the road. Colin Chapman wasn’t wrong.
Very much this. With today’s tech, we could have comfortable long-wheelbase sedans big enough for all occupants to fully stretch their legs out, and plenty of headroom, weighing in around 2,500-2,800 lbs. Just cut all the profit-margin-adding crap.
Weight loss begets more weight loss. It’s a compounding effect.
And my f150 is on 15s and still has enough brakes to lock up the tires on any surface, at any speed. Including when loaded.
Big wheels are dumb
there’s a surprising amount of cars with tiny-ass brakes hiding under giant dubs; it looks like you can shrink by 3-4 inches easy in many cases
Hey, don’t brake shame. They are only ever rolling at drive through speeds anyway…
“Wider tires tend to have higher RR”
I’m a bit confused about this. My understanding is that the size of the contact patch (i.e. the part of the tire that would be flat at any given time) depends on tire pressure and weight placed on the tire, so a wider tire has the same size contact patch, but it is larger side to side rather than front to back. If this is true, wouldn’t a wider tire cause less discrepancy between the belt length and tread length than a narrower tire? If so, shouldn’t there be less heat/energy lost with a wider tire and therefore less rolling resistance, assuming the tires are made of the same material?
(sorry if this was explained above and I missed it; I hope this isn’t a stupid question)
It is true that the contact patch becomes shorter and wider but there is a longer portion of tread that has to transition from round to flat.
Thanks – I think I got it now. The contact patch is the same, but a wider tire has more overall surface area. So per revolution of the tire, more tire touches the ground (and therefore more total tread area goes from round to flat) on a wider tire than a narrow tire. I presume it is more efficient for less tire surface area to deform more than it is for more tire surface area to deform less.
I watched the Tire Rack’s video on LRR before before getting new meats on my hybrid Fusion. Yes, it made a difference MPG-wise in their testing, but nothing I’d call earth-shattering. So while my new tires are not low rolling resistance, they’re also not completely useless at the first sign of snow, a tradeoff I’m more than happy with. I’m still doing my own mileage testing, but with my shorter than average commute, I only fill up once every three to four weeks, so gathering data has been a touch slow.
Larger wheels also put more mass out at a farther radius, increasing the mass moment of inertia. This means more energy to start and stop each time, requiring more power to accelerate to the same speed. Think of the old science experiment with two identically sized cylinders, one solid wood and one a thin wall made of steel. Both weigh the same but the wood one will accelerate down a ramp much faster due to the lower mass moment of inertia. That’s got to play some role here too.
Though once you are up to speed, I would think rolling resistance dominates the losses…
Which is why larger (typically heavier) wheels have more of a negative effect on your city driving, where you have to accelerate from a stop more frequently, while rolling resistance is more of a factor on the highway.
I expected the article to make this point about the additional energy needed to apply rotation to larger (typically heavier) wheels and was a little surprised it seemed to only cover the friction side of the issue. (Apologies if I missed that part)
You are correct but that additional rotational energy is partially recovered by the regen braking in hybrids. There is a net loss of course, but it is less than it would be in a non-hybrid.
I was hoping someone would mention this as I read through the comments, and you did. Thank you. The effect on acceleration is profound. As a general rule of thumb, every 1 lb of rotating mass lost has roughly the same effect on acceleration as losing 10 lbs of non-rotating mass. Circumstances can vary widely from this, but for objects on an automobile, such as rims, tires, and flywheels, this figure will be close.
So I’ve seen people talk about this. It still doesn’t seem like rotating weight would have massive effect on fuel economy. Losing a few pounds from each wheel and tire…… still only adds up to as much weight as a full tank of gas. Or a friend in the passenger seat.
For fuel economy, that is true about rotating weight, especially in conditions of steady state cruising and little stop and go. But for acceleration, the effect is profound. This is the reason bicycles have such light-duty wheels.
And yet, we continue to see cars coming standard with Conestoga wagon wheels because looks > MPG. That bugs the hell out of me. Instead of problematic gimmicks like start-stop technology and variable valve displacement, take a couple inches or two off the wheel diameter and get the same, or more, fuel economy gains.
This also requires more clearance in the vehicle packaging, especially in the front for steering. As the transition from minivans to SUVs has shown: same foot print – less passenger and stuff space. Those big butch ‘I’m not woke’ wheels waste a lot of space.
what’s “variable valve displacement”??
I think VTEC would technically count here. When VTEC kicks in (yo), you’re literally getting more displacement (more opening) of the valves.
You’re missing a major one: Inflation. I have a 1g Insight with 60hp, and if you don’t have LRR it’s a lot harder to stay in lean burn and hit them /r/nextfuckinglevel MPG numbers. However, you can cheat. I’m running VX rims with the narrowest 13″ tires I could find, and then over inflating them. This makes the contact patch more like a bicycle tire, and less like a car tire, which reduces rolling resistance.
However increases braking distances. Be sure to keep dem /r/nextfuckinglevel following distances!
The Insight is so low in mass, that its ratio of mass to contact patch area probably isn’t that far off from the average car, and may in fact be an improvement.
I’m not calling out your decisions here. I just want to promote more hyper miling discussions!
Just to be pedantic, they use silica in tires, not silicone.
I would love to do a range test of an EV with some old military style NDT tires.
Thanks Tom. Text has been fixed
Thanks for that, as I was going to comment that I have been walking past massive bags of silica at the tire plant I’m at today
I don’t understand why a bigger wheel with a smaller tire loses so much range. I would think that the increased wheel size would be offset by the smaller tire side wall height. Just keep in mind that I don’t understand electricity or automatic transmissions.
It’s not so much that bigger wheels reduce range, it’s more that bigger wheels usually come with higher performance tires that have higher rolling resistance.
correct, and nothing was mentioned about rolling resistance and how that effects cornering ability and stopping distances as well as traction when moving forward. Also no mention of unsprung weight and it’s affect on contact patch. Both are side effects of gargantuan wheels with equally wide and sticky rubber bands for tires.
This is a concept I’ve never considered. However I can imaging the difference is negligible. A four thousand pound car with 50/50 weight distribution would have 1k pounds of weight on each contact patch. Even a 100 pound difference per corner (as significant as I can image) of unsprung weight would barely put more tire to the road.
Those larger wheels often weigh more. Add 3 lbs per wheel or 12 lbs in total at the place its not wanted. City driving will show a large difference having to accelerate that extra rotating mass.
In the Volt world, it’s well known that switching to your winter tires costs you 10-15%. The remaining delta is energy used for cabin and battery heating.
Winter tires tend to have high rolling resistance due to their construction and chemistry. This gives them traction on snow but hurts range.
The EU Energy Label for tires has classes based on rolling resistance coefficient. So, as a consumer you could figure out in which rr range your tires would be. Plus wet grip and noise, etc.
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32020R0740
dang.. this was intended for the thread further below. ah well…
Do the aerodynamic wheels vs regular wheels have much of an effect? I removed the aero covers from my wheels at the same time I changed to different tires, so I really messed up my chance to compare efficiency.
Yes there is an impact. Wheels churn up the air quite a lot so smoothing that out with aero wheels can be a benefit.
Hence the reason the original Airflow Chrysler/dodge/desoto products had the rear wheels completely covered and the early Hybrids from Honda and Toyota also partially covered the rear wheels. I suppose if people would buy the style a modern version of the old Nash Bathtub cars would be pretty efficient with the front wheels also mostly covered by body work.
Embrace Moon Discs, there’s a reason they use them at Bonneville. https://www.mooneyesusa.com/MOON-Disc-s/211.htm
Just make sure not to brake too much! 😀
I got a set for my electric Triumph GT6 conversion.
I suspect that the reason Moon eyes are used is more due to the lack of truly Aero rims above a 13/14/15″ size. They arent the best, but they do work.every time I see them I wonder how much drag they create at 150 or 180 mph. They do stick out.. A few years ago Honda spent considerable time making sure the rims on the Insight (1st gen) were the most aero out there. Curiously, Im old enough to remeber when Oldsmobile had some of the most Mooneyes-like.(Olds 1990 88 wagon – and 80-84 Delta sedan).
I believe that Honda took the Insight rim design and expanded it from 13 to 15″ on the Honda Fit and again on the Civic. They are flat, no protrusions with a small lip and vent that creates a minimal airflow to keep air moving inbound, I suspect.
ymmv
Jeep people know that tire changes can make a big mpg difference. going up in width, hight, weight and tread pattern can really change MPG.
Huibert, isn’t it silica, not silicone, being incorporated into tire formulations?
You are correct. Edited!
Basically what I said in my comment on The Morning Dookie, but you made it all science-y and stuff, so yay! And while bigger tires *may* lead to handling improvements, my personal take on putting bigger wheels/tires on an EV it that it’s mostly for the looks, because you almost never hear an EV owner say “I’d really like my car to have less range!”
Also less cushion over bumpy roads and higher chance of blowouts. I *HATE* the large-wheel rubber-band tire look and I hate the way they handle and I hate the range impact. I always choose the smallest wheel (i.e. – largest sidewall) available on any car I buy.
they also don’t want to say, Hey, my 5000 lb EV won’t stop or turn. they might enjoy an occasional smoke show from spinning tires, but probably only when they are trying to show up a Mustang or Challenger.
Thanks for that excellent explanation. But doesn’t rotational inertia have an effect as well? If I remember my college physics correctly (and there’s a strong chance I don’t), a wheel with the mass concentrated farther out from the center will have a higher rotational inertia. So wouldn’t a smaller wheel have a lower rotational inertia than a larger one, requiring less energy to rotate it? Or do the wheel designs distribute the mass so that this isn’t an effect?
Yes, the wheel inertia has an effect but that really only applies during acceleration. Also, in an EV, you get some of that back during regen since the higher rotational inertia means you can get more regen. Lastly, don’t forget that the tire is still there and it has rotational inertia as well. In most cases, the weight of the wheel and the weight of the tire are similar so it’s a trade-off between wheel weight and tire weight.
“In most cases, the weight of the wheel and the weight of the tire are similar so it’s a trade-off between wheel weight and tire weight.”
Eh, I question that logic. If we are assuming a same OD of the tire, just moving to a 1″ shorter sidewall/ 1″ bigger radius wheel loses a bit of rubber sidewall on the two sides of the tire in exchange for a non-trivial increase in wheel rim circumference. The wider the wheel rim, the more weight that is added to the thick aluminum rim. And not only is it more weight, but the increase in distance from the wheel hub will also have an impact how hard the wheel is to rotate from a stop. Meanwhile, the saving in rubber weight is consistent regardless of wheel/tire width since it always comes from just the two sidewalls.
This seems like it would be a non-trivial impact to acceleration. I would expect wheel size would be a significant contributor to differences in city range, despite getting some (never all) of the weight difference back in regen braking. Not saying RR isn’t the biggest contributor, but wheel size should be a contributor.
I didn’t mean to imply that there is no difference. There certainly is, but it is not as much as you might think because a hybrid (or EV) has the ability to recover some of it with regen.
On an ICE vehicle, yes. Because it does not have regen, so that energy gets wasted as heat when braking and isn’t converted back into battery through regen.
Correct
Interesting article. I noticed my Polestar 2 has one EPA range for the Dual Motor, but there are two different tire sizes, both are summer tires from what I recall, different brand. I wonder if they actually get the same range or the Performance package gets less. EPA is 260 miles with or without the performance package.
There is also a performance upgrade available that has the same EPA range…. so we have more combinations possible lol
My assumption there would be that Polestar worked with their tire supplier to make sure the various tire sizes they used have similar rolling resistance.
There does not seem to be published ratings for rolling resistance. Can an ordinary consumer look at the UTQGS ratings on, for the sake of argument, two tires and make a guess as to their relative RRs? Having to rely on a manufacturer’s marketing statement that a tire in their product line is an energy saver seems like a thin basis to make a buying decision.
Agreed, tried to buy more efficient tires recently, but comparing across manufacturers is near impossible. Europe includes an efficiency rating in their standard testing, but of course their tire market is much different than north america, making that data tough to use.
Unfortunately, no. Tire manufacturers don’t generally publish rolling resistance figures. As an OEM you can get this info from them but not as a consumer.
I would think the folks at Tire Rack would start to include those figures (as they may be available) and push for more from the manufacturers. Can confirm our Touring trim Niro gets worse mileage than lower trims due to wider tires… that said, no further drop when switching to winter tires (because Minnesota) as we went to base trim sizes.
It’d definitely complicated buying tires for a hybrid. I have noticed fairly large differences in MPG between tires, but then you also have to factor in the price and treadwear ratings.
I find the inability to get RR ratings very frustrating. Almost everything seems to have an “eco-focus”, including plenty of tires well known to have a fairly negative impact on MPG (Michelin Cross Climate 2). The best you can do sometimes is forum posts.
I really enjoyed that breakdown, an in particular the deep dive into tire contact and tread. David has talked about how skinny his i3 tires are, but now we understand even more why they are.
I had a college professor was fond of saying “There’s no such thing as a dumb question–there’s just dumb people who ask questions.”
Guilty