I just bought a AWD 95 Legacy Wagon with two tires worn bald on the outside of the tread by underinflation, thinking that I could just replace the 2. After spending many hours on line and tracing down threads at this site (and not liking that everyone said I had to replace all 4), I came upon a fellow that quoted from the mysubaru.com web site:
From mysubaru.com: All-Wheel Drive (AWD) vehicles, it is extremely important that the rolling or outer circumferences of the tires be within 1/4 inch of each other. This means that you must physically measure the size of the tire. This is best done with the weight of the vehicle off the tire and at the tread centerline. Also be certain that the tires are properly inflated since this can affect your readings.
If you need to replace just one tire, the same holds true. The measured difference in circumference between the replacement tire and the other tires on the vehicle cannot exceed 1/4 inch.
Depending on the vehicle mileage, it might be better to replace all four tires.
If the vehicle mileage is low and the tires have been rotated and driven at the proper inflation pressures, then you may be within the allowable 1/4-inch difference in circumference.
If the difference between the new/replacement tire and the current tires is within this 1/4-nch maximum, then the tire should be compatible. If it is not, then other tires will need to be replaced to conform to the 1/4-inch rule.
Exceeding this 1/4-inch difference in tire circumference can place unnecessary wear on drivetrain components, possibly causing them to wear out prematurely.
Still not liking what I was reading, I checked at mysubaru.com in the technical advise section, and learned that the above was an exact quote. Basically, the price we have to pay for AWD and going anywhere in any kind of weather is to do 4 tires at a time. Some people get 5 and then rotate them to keep them at the same wear level, then you still have 4 if a rock rips out the sidewall on a tire. I think that I will just have to spend the money.
