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Attention, local speeders: The tire excuse doesn't fly

I have 20-inch rims with low-profile tires on my Chevy Tahoe. My husband believes these cause my speedometer to be off by 3 mph, so that when I am going 55, I am actually going 58 mph. Please tell me whether this is true.

— Linda Hughes

This is an excellent question, Linda. I'm forever getting the "I-just-put-oversized-tires-on-and-my-speedometer-is-off" excuse from speeding drivers, although I've been doubtful that it adds the 15- to 20-mph average that I'm pulling them over for.

So I did some calculating using the good old Pi-times-radius-squared formula, but that got me wrapped up in loops. I thought there must be an easier way — maybe this info is on the Internet — so I Googled tire sizes and speedometers and got a bunch of hits. But, alas, most of them are based on the tire size and you don't mention what size tire you have, or what year your vehicle is.

So I called up the good folks at Les Schwab on Commerce Drive and talked with Jeff about your question. He went to his master tire book and we estimated a "stock" tire for a 2000-2006 Tahoe. I plugged that into one side of the formula and then he gave me measurements for a low-profile tire using 20-inch rims that would fit the same Tahoe as the stock tire. I plugged that into the formula and hit the "calculate" button. The answer was that at 55 mph on stock tires, you'd be going 56.2 mph using the low-profile tires.

Jeff said even if you had a vehicle jacked up high and used really extreme tire sizes it's very hard to throw your speedometer off by even as much as 10 mph. He said exchanging tire or rim sizes usually will mean a differential in speed of less than 5 mph.

Thanks to Jeff at Les Schwab for the tire education. For you speeders out there, forget that excuse, it's not going to fly. If you're interested in the difference that tire sizes make in speed, the Web site I used was www.paspeedo.com/calculator.htm. Have fun.

Dace Cochran, a patrol sergeant with the Jackson County Sheriff's Department, writes a weekly Q&A column on police issues for the Mail Tribune. Have a question for him? Write to Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501, or e-mail cochradc@jacksoncounty.org.