Why Wheels And Tires Make Such A Huge Difference To An EV’s Range And A Gas Car’s MPG

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”.