The Effects of Tire Size on Vehicle Speed

Tire size has the exact same effect as changing the gear ratio in the axle. Vehicles are designed with tires of a certain height. The width of the tire makes no difference, but the height does. Consider that a 27-inch tall tire needs to rotate say for example 400 times in an eighth of a mile while a 31-inch tall tire only has to rotate 320 times in an eighth of a mile. The taller the tire, the faster the vehicle will move for the same amount of RPM, however the speedometer will show a slower speed than what the vehicle is actually traveling. The reason for this is that the speedometer gear is set at the factory to indicate speed based on a 27-inch tire.

Know the long-range implications of changing tires. A taller tire will give better fuel economy on the highway but worse in town, with less acceleration. The speedometer, unless corrected with a different gear for the present size tire, will show an inaccurate number. For example, the speedometer will show 50 miles per hour while the vehicle is actually doing 58. This also affects the distance traveled on the odometer. It will show less distance traveled than the car has actually traversed. It also affects the transmission shift points, which are set up for the smaller tire. Another aspect of taller versus standard size tires is that the taller the tire the softer its side wall and the worse the handling, since the tire has much more flex in the sidewall and the vehicle is much less aerodynamic.

A smaller tire has the opposite affect on the vehicle. A smaller tire will have much better acceleration, be better in town but lose a little fuel economy over the larger tire. That's because it is turning faster or the engine is running at a higher RPM at 60 miles per hour than with the larger tire. The speedometer can be corrected by having the correct speedometer gear installed for the size tire being used.