The car is a 2000 Jeep Grand Cherokee. I don't recall the engine size but it doesn't seem important; it's in constant 4-wheel-drive. Yesterday we had a low tire so we pulled into a well-known shop to see about getting it fixed. Turns out the tire not only had a nail in it but was starting to split in a particular spot. We asked about a replacement tire and the guy told us that we would have to replace all four tires, because if they aren't all the same size and make and tread, the mismatch will tear up the transmission. I asked him how that comes about and he basically said that different treads or whatever will put sideways torque on the transmission and rip it to shreds. To some degree it makes sense, but here's the weirdness: he had no problem putting our spare on the car, even though it's a different brand than the other tires, has a different tread, and is brand new where the others are worn down. Was this guy feeding me a line of crap to sell me more tires, or am I missing something?

The spare is just that. A spare "temporary" tire. Also because the ire belonged to you its not his problem. By replacing only one tire without forewarning you he would have accepted liability for a possible transmission problem. With 4 wheel drive there can be a torque difference between makes of tires.

The spare is just that. A spare "temporary" tire. Also because the ire belonged to you its not his problem. By replacing only one tire without forewarning you he would have accepted liability for a possible transmission problem. With 4 wheel drive there can be a torque difference between makes of tires.

How likely is it and how long would it take to do real damage?

Really hard to say because its based on the existing components. The effect of bieng in full time 4 wheel drive in nature causes binding to the system and stress anyway. Also depending on the type of tires and the pavement its driven on all add to it. Its just always better to be safe then sorry and get the tire replaced or fixed soon.

Really hard to say because its based on the existing components. The effect of bieng in full time 4 wheel drive in nature causes binding to the system and stress anyway. Also depending on the type of tires and the pavement its driven on all add to it. Its just always better to be safe then sorry and get the tire replaced or fixed soon.

Sounds as though, with the constant 4-wheel-drive setup, it's pretty well inevitable in any case, so if the spare is the same size as the others it shouldn't matter too awful much. Or have I misunderstood?

Its not the actual size of the tire but the tread pattern. The idea being if it is different from the others then it can cause a torque differential. The rating on the tire should give you a clue. It should be on the side of the tire.

Ref:

Tire Tech Information - How to Read Speed Rating, Load Index & Service Descriptions (http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35)