Here are four simple reasons why one may want to consider a new set of four tires when the time comes.
1. Reduce Uneven Front-to-Rear Wear
For increased stability and handling, particularly in wet or slippery conditions, buy tires in sets of four and rotate them often. If you don't buy four, always install the best tires on the rear of the vehicle. If the front tires are worn more than the rear tires, rotate them side-to-side instead of installing the worn ones on the rear.
2. Protect Your Transmission & Transfer Case
All-wheel and four-wheel drive vehicles require that all four tire circumferences remain very close to the same measurement. It is best to replace all four at the same time.
3. Maintain Accurate ABS Brake and Traction Stability Controls
Today's modern vehicles measure the rotational speed of each wheel as you drive and compensate engine performance and braking when one or more tires are traveling at a different rate of speed. Prolonged driving with uneven tire sizes could effect these readings.
4. Keep your vehicle looking great!
If you are unable to replace all four tires, here are some general guidelines presented by Cooper.