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Old 02-08-2010, 08:20 AM   #1
jrcase
 
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Different rim sizes = speedometer reading wrong?

With the 2SS offering both 20" and 21" wheels, it seems to me that the speedometer will be inaccurate on one of the packages. What does GM use as a baseline calibration for the speedometer, the 20" or 21"?

Also, does the V6 18" use the same calibration? It seems to me that with all the different sizes, someone's speedometer is going to be off.
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Old 02-08-2010, 08:22 AM   #2
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rim size has nothing to do with the speedometer, they all have the same overall tire height
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Old 02-08-2010, 08:29 AM   #3
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Quote:
Originally Posted by camaro1 View Post
rim size has nothing to do with the speedometer, they all have the same overall tire height
I was thinking that too. The rubber sidewall is shorter to compensate for the larger rim but overall the total height should be the same.
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Old 02-08-2010, 10:48 AM   #4
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Old 02-08-2010, 11:04 AM   #5
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Quote:
Originally Posted by camaro1 View Post
rim size has nothing to do with the speedometer, they all have the same overall tire height
You are right. I read that GM used the same outer diameter on all of the tires so that they would only need one speedometer.
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Old 02-08-2010, 11:33 AM   #6
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Speedometer goes off revolutions of overall diameter of rear wheels/tires. Go to a bigger rim, you put a shorter sidewall tire on to get the same diameter as before. However, if Chevy wanted to create different diameters between the 18's, 20's and 21's with their tire sizes they could have b/c a speedo can be calibrated easily to correct tire sizes.
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