Larger Tires = Lower MPG

[rustyfoot7]

I'm trying to research why larger tires would decrease a car's mpg even though larger tires can travel a greater distance with each rotation of the axle. Most of the reasons given on the internet claim the added weight of a larger tire will cause the lower mpg. This doesn't seem like a very good reason since the added weight of larger tires would be insignificant compared to the weight of the car. Also, tire/wheel weight can be manipulated by changing width and using more expensive and lighter materials. There has to be another reason.

 

Could it be that the force a larger tire puts on the road is less than the force a smaller tire puts on the road (with an axle of equal size)?

 

I'm hypothesizing that because the torque of the axle, a small tire and a larger tire should be the same but the force from their edge would be different. (Torque = Force x radius)

[swansont]

If you e.g. double the size of the tire, the moment of inertia (which varies as MR^2) will increase. The radial term will increase x4 because it depends on R^2, and the mass will also increase (x 4 in a naive assumption that ignores scaling effects and other details). The tire can slow down by a factor of 2 to give the same linear speed, but that only cancels part of the increase. So you have more energy tied up in the tires. You add this every time you speed up and lose it to the brakes every time you come to a stop. If the "at speed" losses are proportional to the energy, this also represents more loss. But "city driving" mpg should definitely take a hit.

 

Bigger tires also can have more rolling resistance, which will decrease mpg.

 

There's also the possibility that the engine is less efficient at the speed it's running at for the new tires when traveling at a given speed.

[rustyfoot7]

Thanks for the post! I didn't think of it in terms of inertia. Angular acceleration of the tires would decrease with increasing inertia. (Torque = Inertia x angular acceleration) However, for highway driving, where very little acceleration is needed, larger tires should give better gas mileage. I can see why in city driving larger tires would suck because of constant braking and accelerating.