If you purchased your first all-wheel-drive car and experienced a flat tire that can’t be repaired, you know what tire-related sticker shock is like. It’s not the individual price of each tire that will make you queasy; it's the requirement that all four tires be replaced. Yes, I said all four tires.
The problem is that the wheel speed of the tires is monitored to detect slippage that would activate portions of the all-wheel-drive system. This is accomplished by using additional differentials or viscous couplings (sometimes both) to allow for differing wheel speeds experienced when cornering or encountering loose pavement and momentarily spinning a wheel.
When the circumferences of tires are not matched, the number of times the tire revolves per mile can vary by tire. This can occur when tires are replaced with no attention paid to the qualities that affect circumference like size, type, and tread wear. When this happens, the system described above is asked to work 24/7--and soon, heat-related damage can occur.
Longtime transmission specialists have advised that the industry standard is that one-quarter of an inch of variation of the tire circumference is tolerable. This measurement is taken by using a small tape measure and hooking it into a cleat of the tire tread and rotating the tire to record the circumference. It is assumed that every effort is made to match the replaced tire with the remaining tires on the car. This means that the same brand, tire model, size and tread design should be used. The inflation rate of all four tires should be uniform as well.
Recommendations about replacing tires on AWD and 4WD vehicles vary by car manufacturer, so a thorough reading of the owner’s manual is required. Tire rotation and inflation also assume increased importance as does the need to work the full-size spare into the equation (if provided). The trouble with tire management on these specially equipped vehicles is that the stakes are high--as anyone who has had a transmission repaired can attest.