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Old 01-21-2010, 12:31 PM   #1
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Default Speedometer Off?

i was crusing down the highway at 75 one day and look over and im passing a civic whos speedo said 80 so i had my girlfriend follow me in her 08 Mazda 3 stock. while i was crusing at 75 i called her up and had her pace me. she said her speedo said around 80. but whenever i pass those speed checkers that the cops set up it said that my speed was correct. but that was going 35. i have put 18" rims so could that be the problem and it cannot be detected going at low speeds?
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Old 01-21-2010, 12:40 PM   #2
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It's not about the wheel diameter, but rather the overall tire diameter.

The stock tires on these cars are 25.66" tall. If your current tires are significanty taller, then this could be the reason.

What size tires are you running?
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Old 01-21-2010, 01:20 PM   #3
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245/40/18 in front
285/40/18 in back
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Old 01-21-2010, 01:42 PM   #4
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Quote:
Originally Posted by my bandit View Post
285/40/18 in back
Ah, that explains it then.

Stock 245/50/16 or 275/40/17 is 25.65" and 25.66" tall respectively.

Your tire (285/40/18) is 26.98" tall. That's a big difference.

So with those specs, the calulator at Discount Tire says that if your speedo is showing 75mph, you're actually going 78.89mph.
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Old 01-21-2010, 01:42 PM
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16, 17, 2002, 245, 285, 40, 50, calulator, camaro, ls1, ls1tech, speedo, speedometer, speedometeroff, z28


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