Have you experienced a tire failure on a vehicle with all-wheel drive? Unfortunately, the complex systems that deliver power to all four wheels are sensitive to mixing tires with different overall diameters. Simply installing one new tire runs the risk of drivability problems or expensive damage, so you must follow some guidelines when replacing one tire.
Some manufacturers have set perameters for matching the tires used on their four-wheel drive and all-wheel drive vehicles.
- Audi: As published in their vehicle owner's manual, "rolling radius of all four tires must remain the same" or within 4/32-inch of each other in remaining tread depth
- Porsche Cayenne: Within 30% of the other tire on the same axle's remaining treadwear
- Subaru: Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth
If the idea of having to replace all four tires sounds too expensive, there is another option: shaving. Tire Rack can reduce the tread depth of a new tire via the removal of tread rubber on a specialized machine that operates as a tire lathe. Prices vary based on how much tread depth must be removed, but is normally about $25 per tire. While shaving off perfectly good tread may seem counterintuative, consider the additional costs of having to replace all four tires (or worse, the driveline damage from mixing tires with different depths!).
You can read more about this in "Mixing Tires."