I am listening to Car Talk on NPR. One caller said her husband claimed that putting larger-radius tires on the car would improve gas mileage. The hosts agreed, saying that the car would move farther with each rotation of a tire. The cautioned, though, that there would be a loss of power.
This doesn’t smell right to me but I don’t know what empirical data shows. One factor of gas consumption has to be the physical work done, or force integrated over distance. At low speeds this force is primarily that needed to overcome friction, and at higher speeds it is primarily that needed to overcome air resistance. Increasing tire diameter does not mitigate either of these factors. Another factor is rolling resistance between the tire and the road, also unchanged by tire diameter alone.
Increasing the tire size seems equivalent to raising the gearing. There I would agree with the brothers that it would reduce power. However, raising the gearing would improve gas economy only dependent on engine performance characteristics. I would expect that the engineers design a particular car to optimize these factors and messing with any of them will de-optimize some aspect. Otherwise it would be a no-brainer to raise gearing or increase tire size on every car to improve gas mileage.
Aside from theoretical arguments, is there any data on this?
As usual, those two…people are only partially correct.
One advantage that larger tires have is decreased rolling resistance on real roads. Think of the angle where the tire meets the pavement, and you can see that as the tire diameter increases, the actual angle of approach over road irregularities changes.
Other than that, your point about the gearing/tire speed and being tuned to the engine’s operating characteristics is very valid.
Larger wheels have lower rolling resistance even on ideal (perfectly smooth) surfaces. It’s well known in the bicycle industry. The road compresses the tire at the front end of the contact patch, and the tire pushes back at the road on the trailing end. But these forces are not equal, becasuse some energy is lost due to internal friction of the rubber (hysteresis loss). This means the road applies a torque against the turning wheel, and the smaller the wheel, the larger the torque because the front end of the contact patch is farther ahead (by angle). However if you make the wheels too large, you increase the weight and air drag of the wheels themselves, negating the reduced rolling resistance.
One more thing that would have to be taken in to consideration is the by changing the tire size you are in effect changing the gear ratio of the drive system of the car. This would make the engine run at a lower RPM, that may or may not cause an increase in fuel economy. There would be some point in the RPM curve of an engine that it runs the most efficiently, the only way you would get better fuel economy is if the change in the gear ratio would changes the engine RPM to a more efficient value. It is just as likely that the change you make would make the engine run less efficient.