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      03-31-2012, 03:45 AM   #1
atruelunatic
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Question: Speedometer difference

I know that speedometer is generally off by about 4%, but after changing to AD08's (285/30/18) from the stock PS2 (265/45/18), the difference increased by a whole lot. Cruising on the highway, the difference is roughly 7mph. I was fine before when I had a 2 mph difference, but 7 is sort of too much. I'm always caught off guard now when I notice my speedo at 87-90MPH and then remember I'm going 80-82 MPH. Any way to fix this? And does this effect my odometer reading by much?
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      03-31-2012, 03:44 PM   #2
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When I had the stock rims/tires on I averaged about 6% off of the speedometer compared to a GPS unit, in fact I tested this across several GPS units, TomTom, iPhone 4S Google Maps and one which I forget the name of. I doubt there is any way to correct this error unfortunately.

With regards to your odometer that is a very good question. Why don't you set cruise to 40mph and reset the reading, then use a timer to see if the miles add up in a period of say 3 minutes. If it is at 2 miles then obviously the odometer goes according to the speedometer and not real world speeds.
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      03-31-2012, 04:19 PM   #3
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Tire Rack has spec tables for all the tires they sell where you can find tire revs per mile so that you can find out what the impact of different tire sizes will be before you leap. My experience is that, unlike the speedometer, the odometer is fairly accurate with the standard tires (your results may vary). You can determine the error you now have by using a GPS to compare its distance traveled information with your odometer over say a 10 mile run.
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      03-31-2012, 06:42 PM   #4
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I asked the same question a couple years ago and nobody had a solution. Your best bet is to go up to a 295/30 as the diameter is very close to stock.
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