Why would GM's spare wheels be an inch smaller than the regular wheels on the car?

Dear Car Talk

Dear Car Talk | Jan 01, 2003

Dear Tom and Ray:

I have a question that is driving my husband and me crazy. The wheels on our 2003 Chevy Tahoe Z71 are 17-inchers. But the spare has a 16-inch wheel. The sales people are scratching their heads as to why. I e-mailed both Chevrolet and GMC, and they gave us a bunch of double talk, saying they don't have any information available to them and referring us to the "parts" department at the local dealership. We're hoping you can get an answer for us. -- Martha

TOM: C'mon, Martha. You've seen one wheel, you've seen 'em all. What difference does an extra inch make?

RAY: Actually, we e-mailed GM, and it provided an answer for us. It says that although the spare wheel IS an inch smaller in diameter, the tire is, in fact, bigger. So, overall, the wheel/tire combinations are the same size. In other words, they have the same outer circumference, so they all travel the same distance per revolution of the axle. Therefore, no damage is done if you drive with the spare tire installed.

TOM: GM says there are two reasons for providing a 16-inch steel wheel as a spare. One is that some 17-inch wheels are difficult to fit in the given spare-tire storage space.

RAY: But if the overall wheel and tire combination is the same size, what difference could that make?

TOM: I don't know. Maybe Martha will e-mail GM and get us the answer to that. I'd have to guess it has to do with the shape of the spare-tire well. Since it was originally designed for 16-inch wheels, maybe the wheel itself is difficult to secure if it's bigger than 16 inches, even if there's enough room for a bigger tire.

RAY: OK. The other reason it gives is that the 16-inch spare is a steel wheel, while the 17-inchers on the truck are aluminum-alloy wheels. And since the spare is stowed under the vehicle, GM says a steel wheel is less likely to receive cosmetic damage while under the car for a long time.

TOM: And we'll add one more very important reason that GM didn't mention, Martha: A smaller steel wheel is cheaper.

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