Will changing my tire size result in a messed up speedometer?

Dear Car Talk

Dear Car Talk | Apr 01, 2001

Dear Tom and Ray:

I have a 1996 GMC Suburban. The original tires were 235-75-16s. I replaced them with 255-70-16s. Will the size difference change my speedometer reading? -- Lew

TOM: Well, GMC doesn't recommend 255-70-16s for this truck, Lew. And throwing off the speedometer reading is probably the least of its concerns.

RAY: The main concern when you change to an unauthorized tire size is how it will affect the vehicle's handling and safety. When cars and trucks are tested for their handling and load-carrying abilities -- and things like their propensity to roll over -- they're tested only with their original-equipment tires.

TOM: Other sizes of tires might turn out to be OK, but the manufacturer simply hasn't tested them. So there's no way to know exactly how the new tires you bought will affect your Suburban's behavior on the road.

RAY: Although GMC hasn't tested 255s on this truck, it does caution that you might have trouble with the wider tires rubbing against the wheel wells. So that scraping noise you've been hearing is NOT your neighbor's garbage can that you ran over six weeks ago.

TOM: And since your replacement tires have a smaller aspect ratio (that's the "70" number, and it means that the new tires are slightly smaller in diameter), your speedometer will read faster than your truck is actually going. So the next time you lie to the police officer and say you were only going 70 when the speedometer said 75, you might accidentally be telling the truth!

RAY: Here's the lesson in this: Either buy replacement tires with the exact same specifications as the originals, or check your owner's manual, your manufacturer's Web site or ask your dealer's service department for a list of acceptable replacement tires and tire sizes.

TOM: Certain tire dealers are notorious for selling you whatever they have in stock, whether it's approved for your vehicle or not. So your best bet is to call ahead and ask them if they have the exact type and size of tire you're looking for. Otherwise, you're likely to get the line, "Well, we don't have those, but these 395-25-19s will fit your car just fine ... and they'll give you a better ride, too."

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