Piston Slap: Financial Traction, AWD Distraction?

John writes: Hey Sajeev, Since you requested goofy questions for Piston Slap, I've got one: A friend of mine told me that her dad had a Subaru (I don't know what particular model). He had one of the tires blow out, and even though he knew he should either replace all four tires, or have the new one shaved down to match the other three, he decided to risk it and just use the one new tire as is. Consequently, the all wheel drive system got messed up due to the ever so slight mismatch in tire diameters. Is this for real? I've never heard anything like that before. Sajeev answers: I didn't request "goofy" questions, per se…but let's not split hairs. And this is far from a goofy question. Because this problem is for real, a good explanation is here. And it's not an "ever so slight" mismatch with the tires, if the ¼" circumference discussed on UltimateSubaru.org is valid. I regularly dissuade people from buying AWD cars, unless they live in colder climates where municipalities simply can't regularly plow all their streets.  Which is a lot of the country, but not a lot of the population. AWD systems are heavier, thirstier and cost more to buy. For the long term owner, they cost more to keep functional. And for anyone who loses one tire from a quartet that had a lot of life beforehand, things get real ugly. This is almost as silly as fretting over horsepower figures when wide-open throttle is rarely applied. I will try to remember this letter the next time someone asks my opinion on an AWD vehicle, when I know that someone doesn't need it.  And that's not a slam on Subaru or Audi, at least not intentionally. The concept of shaving down a perfectly good tire will certainly get some "financial traction" in people's minds! Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you're in a hurry. from The Truth About Cars http://www.thetruthaboutcars.com

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