Do Tires Affect Gas Mileage?
When winning means a few hundredths of a second, nothing is too small to be ignored.
I’ve talked a lot about energy and the importance of using it as efficiently as possible in racing. Gasoline provides a certain amount of stored (a.k.a. potential) energy. Everyone gets the same amount of energy. Winning lies in part in how efficiently you can transform the potential energy of the gasoline into kinetic energy (also known as speed).
We’ve talked about friction — anywhere two moving parts touch, you have to use some of the energy from the gasoline to overcome their resistance to rubbing past each other. Friction robs a car of speed.
There’s another way energy is used up: Deformation. Also known colloquially as “Squishing”. Take a gander at the video below.
This is a common example we use in physics classes to illustrate the conversion of energy from gravitational potential energy (energy of position) to kinetic energy (energy of motion). The ball starts out still, but raised to some height. Its energy it entirely potential. As it falls, it loses potential energy and gains kinetic energy.
If the world were perfect (a phrase that should raise a red flag), the ball would return to it’s original height. All of the kinetic energy would be converted back into potential energy. Hopefully, you noticed that it didn’t. Run the video one more time and look at the overlay (the yellow circle) as the ball hits the floor. I wanted to emphasize what happens when the ball hits the floor.
It squishes. Deforms. Whatever. It changes shape.
Changing shape requires energy. That energy can’t be converted back into potential energy, which is why the ball doesn’t bounce back to its original height. (Note that there are other places where we lose energy. For example, the ‘plonk’ sound when the ball hits the floor takes energy to create, too.)
In some cases, the fact that it takes energy to deform something is good. For example, some areas of a car are designed to crumple more easily than others so that the energy of the moving car is used to smush the car and not transferred to the people inside. These are cleverly called “crumple zones”.
In other cases, deformation is not such a good thing. And one place where deformation happens a lot is in the tires of a car. Look at a tire lying on the ground. It’s round. When you put that tire on the car, the weight of the car deforms the tire, creating a flat spot — a deformation. I’ve embedded a video below from Goodyear — it’s part of their modeling package for simulations and it shows how the tires continuously deform as the car moves.
Every time the tire deforms upon hitting the ground, then springs back, you lose energy.
This is called rolling resistance. The United States Department of Energy estimates that 5% to 15% of the energy contained in the gasoline goes to overcoming rolling resistance in passenger cars. In big trucks, that number can be as high as 15% — 30% because the tires are wider and there are a lot more of them.
To put this in perspective: If you lose 20% of your energy to rolling resistance, that means that one out of every five fill-ups is used entirely to overcome the rolling resistance of the tires. (Side note: Once you consider all the friction, all the rolling resistance, the energy used by cooling units, only 12%-20% of the energy contained in the fuel is used to actually make a passenger car move.
If you go to a tire store (as I did recently, thank you potholes), you’ll find advertising displays that talk about low rolling resistance tires. The grab in these ads is that low rolling resistance tires save you money by decreasing the amount of energy lost to the tires.
This is actually a thing. They run tests and studies to prove that it is true. Now, of course, this only works if you keep your tires properly inflated. Underinflated tires on a car deform a lot more than properly inflated tires. You can verify this for yourself. As you let the air out of a ball and measure how far it bounces back after dropping it, the more underinflated it is, the lower that bounce-back will be.
Ideally, Goodyear would provide tires with exceptionally low rolling resistance; however, rolling resistance is linked to other parameters. A harder tire doesn’t deform as easily, so it will have lower rolling resistance; however, we also know that harder tires don’t wear much and don’t offer a lot of grip. Softer squishy tires give good grip — but have have higher rolling resistance.
Goodyear (and every other tire manufacturer) does research to find ways to decrease rolling resistance without sacrificing grip. A lot of their work is on the tread compounds. Tread compounds are super top-secret recipes with a huge number of ingredients that include everything from rubber to carbon and/or silica nanoparticles.
Now in NASCAR, all teams get the same tires — and teams are not allowed to make any changes to the tires — at all. So although the tires do affect the fuel mileage, every team gets the same equipment. If Goodyear comes up with a way to make a lower rolling resistance tire, everyone on track benefits.
Small Effects Add Up
You may be thinking that we’re talking pretty small numbers. We are, but they accumulate. If there’s something you can do that save you a penny every mile and you drive 50,000 miles a year, that’s $500. On a larger scale, research indicates that reducing the rolling resistance by 10% would increase fuel mileage by 1%. That’s tiny, right? Well, in 2014, the U.S. used 136.78 billion gallons of gasoline. Decreasing that by 1% saves over a billion gallons of gas. Little things add up — as I keep telling my husband when he doesn’t turn off the light when he leaves the room.