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Discussion Starter · #1 ·
Is there a way to correct the speedometer for different tire sizes using the GPS method? In other words, you figure out how fast your car is going with a GPS device compared to what is being displayed on the speedometer and then input a compensation factor to adjust the speedometer.

Or is the only way to do it through input tire revolutions per mile? I've already done it that way, but it isn't as accurate as I would like.
 

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Seems if revs. per mile works you could adjust that until it matches at hwy. speeds.
 

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If your tuning with me just let me know its a free adjustment , if not call your tuner and get them to make the change in the tune for you

Tom
 

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Discussion Starter · #4 ·
I fixed it. Had to go to Montgomery today, so I brought my GPS with me to compare speeds. I was actually going 4 mph slower. Adjusted the revolutions per mile to 890 using the tuner. It is now dead on.
 

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Depends.
I adjusted mine with rev/mile and gear ratio's ... ratio's were off ... 60mph was dead on but would vary higher or lower than.
 

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Discussion Starter · #6 ·
Depends.
I adjusted mine with rev/mile and gear ratio's ... ratio's were off ... 60mph was dead on but would vary higher or lower than.
I based mine on 70 mph, as that is the speed limit on I-85. I also compared my speed with my GPS after retuning and it seems to be accurate at other speeds as well. I compared in a 45 mph zone and when I was going about 65 on the interstate when I was stuck behind a tractor-trailer.
 

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Yep, I do it all the time for my Focus when I switch tires (winter/summer). Make sure to get a GPS reading on a straight level road obviously. (SCT SF3) Nice to know the speed you get on your speedo is real....
 
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