While motoring experts recommend using four same-sized tyres on a car, and specifically the type best suited for the vehicle, exceptions are allowable within certain limits. Within these limits and especially when the limits are exceeded, problems can occur -- the effects proving as harmless as throwing off speedometer readings or as dangerous as creating steering, braking and handling issues. Damage to the vehicle can also result from tyre variations.
Tyre size can vary by width and height, and although a specific tyre is usually recommended for a vehicle to perform optimally, there is a window of sizes that will work safely -- roughly a 3 per cent change in size for cars and 15 per cent change for four-wheel drive vehicles. Within this window, effects are felt mostly in the steering, braking, acceleration and overall driveability. When different width tyres than those recommended are used, the driver may feel the car pull in a certain direction or notice differences in turning and braking. The most dramatic effect of tyre width comes in inclement weather, like snow. Wider tyres don't perform as well in slippery conditions. When taller tyres than recommended are used, steering will seem sloppier and the car may take bumps more abruptly. Also, it might take longer for a vehicle to get up to speed and stop. Shorter tyres yield tighter, more responsive steering but also decrease ground clearance, which can cause other problems. Outside of the recommended tyre size window, the negative effects magnify and can create an unpredictable and unsafe driving environment, especially in emergencies. Also, tyres that are too wide can wear out the wheel bearings and cause rubbing in the wheel wells on turns.
Taller or shorter tyres than recommended will throw off the vehicle's speedometer readings. Shorter tyres will make the speedometer read faster than the vehicle is actually going, while taller tyres will do the opposite -- the speedometer will read slower. In addition, fuel mileage can change due to the rate the tyres are moving -- smaller tyres get poorer mileage.
The most serious effects occur when mis-matched tyres are used on a vehicle. Not only can this dramatically change the handling characteristics of a car by throwing off the weight distribution of a vehicle, it can stress and burn out the differential or the transmission, especially if tyres of a different height are mixed and the location of the oddball is on the front of a front-wheel-drive vehicle or the rear of a rear-wheel-drive vehicle. Once again, outside the "safe tyre size window" effects magnify.