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Questions and Answers

Volvo AWD

Q. Sir; I here is a question that no-one seems to be able to answer directly.

I have the following:
  • 1998, Volvo V70 XC AWD
  • V5
  • automatic transmission
  • 24200 Miles
  • Fuel Injection
  • ABS brakes

I have recent been told by Volvo's Service center that, since my car is an AWD; and since my tires have 'some' thread wear on each of them- that is I want/ need to change a single tire- that it CAN'T be done. Only option is to change ALL four tires. If I only change one tire, Volvo claims that it will burn my AWD system and require some costly repair jobs.

Can you tell me if this is true or not; any good advice is appreciated as well. I'm concerned, because my warranty is due to end in 6 month.

Thanks,
Lee

A. Well Lee, I always felt a direct question deserves a direct answer. As soon as I find one, I'll give it to you. Only kidding.

What the dealer is telling you is, essentially, correct. Here's the reason why.

In an AWD car, all four wheels are powered, just like a 4WD vehicle. When you have all four wheels powered, tire size becomes important, not just the size specified for the car, but the actual circumference of the tire. Lets say the rear tires are brand new and the front tires have, say, 15,000 miles on them. The new tires may have, for sake of illustration, a circumference of 100 inches. The older tires may have a circumference of, say, 99 inches. Not a big difference on the surface. Now the smaller tire has to turn faster in relation to the larger one so now the tires are turning at different speeds. Not a problem in a standard, two wheel drive car because two of the tires are just going along for the ride. But in an AWD car this causes a bind in the drive train since there is no provision for tires that turn at different speeds..

As I said, a one inch difference is not a lot, but as you drive the car, the difference is constantly compounded. The more distance you travel, the tighter the bind. Now this bind puts a strain on the drive train and it will get to the point where something will give. Since taking it out of AWD to relieve the binding is not an option, something in the drive train will let go. And Murphy's Law stipulates that the most expensive part of the drive train will let go.

Ideally, all four tires should not only be replaced as a set, but the circumference of the new tires checked to insure the circumferences are indeed the same. This is why tire rotation is so important on an AWD vehicle. In fact, I recommend my customers with AWD's to rotate the tires in the modified X pattern every 5,000 miles. This insures that all the tires wear evenly and prevent any binding in the drive train.

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