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For big tires, when the rate of speed increases, does the difference in my speedometer vs. actual speed increase, decrease, or remain the same?

MA of Reader:

I have a Toyota truck that has a lift on it and tires that are a little bigger than the factory tires were. The other day I passed one of those radar machines that tell you what the speed limit is and what your speed is. My speedometer said I was doing 40 mph, and the machine said 44 mph. I know my speedometer is off because of the bigger tires, but my question is this: when the rate of speed increases, does the difference in my speedometer vs. actual speed increase, decrease, or remain the same?

-- LB of Poway

Car questions. Phooey. Can't think of a subject that generates more heat and less light than car questions. Get two guys, a case of beer, and a Ford with bad valves together in one place, and in half a day you'll have two guys, a car that's still broken, and a fistfight. Guaranteed. But since the only high-risk sport I participate in is Xtreme Researching, I'll give this one a shot. As you know, your speedo will always lag behind your actual speed because it hasn't been recalibrated for the longer distance you travel on your larger-circumference tires. There is a formula you can use to refigure your actual speed, using your new-versus-old tire size. It gives you a percentage value that you can apply to your speedo reading, no matter how fast you're going. The ratio remains constant.

My car experts actually suggest you ignore the formula, which some say isn't really accurate for every set of tires and every vehicle. They recommend you take the LBmobile out and run a timing check on a flat road with mile markers. (Obviously, you should ignore your odometer since it's lagging too because of the new tires.) Travel a constant speed between the mile markers, time it with a stopwatch, divide 3600 by the number of seconds it took to cover the mile, and that will give your actual speed at that speedometer reading. From that you can figure the percentage of correction for your truck. Or you can stop being so cheap and get a new gear for your speedo cable (or reprogram it if it's digital). Or forget the whole thing and take the Ma Alice approach to speed regulation. If you're getting a ticket, you're going too fast; if people are honking at you and flipping you off, you're going too slow.

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MA of Reader:

I have a Toyota truck that has a lift on it and tires that are a little bigger than the factory tires were. The other day I passed one of those radar machines that tell you what the speed limit is and what your speed is. My speedometer said I was doing 40 mph, and the machine said 44 mph. I know my speedometer is off because of the bigger tires, but my question is this: when the rate of speed increases, does the difference in my speedometer vs. actual speed increase, decrease, or remain the same?

-- LB of Poway

Car questions. Phooey. Can't think of a subject that generates more heat and less light than car questions. Get two guys, a case of beer, and a Ford with bad valves together in one place, and in half a day you'll have two guys, a car that's still broken, and a fistfight. Guaranteed. But since the only high-risk sport I participate in is Xtreme Researching, I'll give this one a shot. As you know, your speedo will always lag behind your actual speed because it hasn't been recalibrated for the longer distance you travel on your larger-circumference tires. There is a formula you can use to refigure your actual speed, using your new-versus-old tire size. It gives you a percentage value that you can apply to your speedo reading, no matter how fast you're going. The ratio remains constant.

My car experts actually suggest you ignore the formula, which some say isn't really accurate for every set of tires and every vehicle. They recommend you take the LBmobile out and run a timing check on a flat road with mile markers. (Obviously, you should ignore your odometer since it's lagging too because of the new tires.) Travel a constant speed between the mile markers, time it with a stopwatch, divide 3600 by the number of seconds it took to cover the mile, and that will give your actual speed at that speedometer reading. From that you can figure the percentage of correction for your truck. Or you can stop being so cheap and get a new gear for your speedo cable (or reprogram it if it's digital). Or forget the whole thing and take the Ma Alice approach to speed regulation. If you're getting a ticket, you're going too fast; if people are honking at you and flipping you off, you're going too slow.

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