Why you should replace all 4 tires on an AWD vehicle at the same time
The general rule we follow when determining if we can replace just one tire, two tires or all four tires depends on tread depth. Tread depth can’t vary more than 2/32 from side to side on any vehicle. Tread depth can’t vary more than 2/32 from side to side or front to back on AWD vehicles. The reason why tread depth can’t vary more than 02/32 is friction. Tread depth on a new tire is typically around 10/32. The amount of tread depth a tire has left determines the total diameter of the tire. A tire with a smaller diameter must spin faster and have more revolutions to travel the same distance as a larger diameter tire. So if you have a tire with 07/32 of tread on the left front and a tire with 10/32 on the right front of a vehicle, the left front will be turning at a faster pace than the right to cover the distance.
On a front wheel drive vehicle, the two front tires are connected by cv axles and a transmission/transaxle. The transaxle is designed to allow one tire to rotate faster or slower than the other side for short amounts of time. Instances like sharp turns or even traction loss on slick surfaces. They are not designed to allow uneven rotation for extended amounts of time. It builds up a lot of friction on the internal parts of the transaxle which leads to high heats and likely failures.
On a rear wheel drive vehicle, the two rear tires are connected by axles and a set of gears in the differential. The gears in the differential designed to go forward and backward are fairly large and designed for a full load. The gears in the rear differential that allow one rear wheel to spin at a slower pace than the other are fairly small and not designed for constant load and friction. These small gears (spider gears) are only designed for use when turning the vehicle. If you mismatch tread more than 02/32 from side to side on a rear differential, those small gears will engage he whole time driving rather than just when turning. This creates a lot of friction and heat. This will eventually lead to a major failure of an expensive differential.
On an AWD vehicle, all four tires are connected by cv axles, transmission/transaxle, differential and often a transfer case/power transfer unit. With all 4 tires being connected, it means that a variation from left to right or front to back on an AWD will lead to the same friction, heat and possible failures of these expensive components. Therefore, it is very important to take great care of the tires on your AWD vehicle. Rotate regularly to keep tire wear even from front to rear. If you do have premature failure of one or more tires, be sure to take the proper steps to determine if you must replace all 4 or not. Error on the side of caution as tires are way less expensive than the drivetrain components that can be damaged by running mismatched tires.