A tire with the wrong inside diameter may not properly fit the rim, while a tire with an incorrect outside diameter may affect the accuracy of the vehicle's speedometer and odometer. Additionally, because the traction control, torque and gearing settings depend on the distance the wheel travels in one revolution, a tire with the wrong outside diameter may also affect the proper function of these critical systems.
However, these pitfalls can be avoided by maintaining the overall size of the wheel when purchasing rims and tires. For instance, if the new rims are larger than the original ones by 2 inches, the height of the new tires must be 2 inches shorter. Keep in mind, though, that the height of a tire is not directly specified and must be calculated from its aspect ratio, or the proportion of its height to its width.
By carrying out these calculations, and buying the correct combination of rims and tires, a driver can even enhance the way a vehicle looks and drives. In fact, upsizing, or using a larger rim, can improve the car's braking performance, cornering grip and steering feel. And downsizing, or using a smaller rim, is useful when buying snow tires, as they tend to be less expensive and more effective on snow and ice in narrower sizes.