Changing the diameter of a vehicle’s tire can drastically affect every aspect of its performance, from acceleration, to braking, to fuel economy. From one extreme to the next, a vehicle can gain or loose as much as 25 percent of its fuel economy, just by changing tire diameter. There are a number of factors that can affect this: The engine’s torque curve, vehicle weight, aerodynamics and gearing.
The most popular notion about how tire size affects fuel economy is that a tire with a larger diameter will cover more ground per revolution, thus reducing overall gearing and causing the engine to run at lower revolutions per minute (RPM). Some assume lower RPM equates with less fuel used to travel the same distance. Though this idea does have its merits and is true in some cases, the effect is hardly universal and varies greatly depending upon the vehicle and the increase or decrease in diameter.