Correct Speedo After Gear Swap

Big Q

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Recently swapped to 3.73's from 3.31 on my 14 GT M6. I flashed the stock tune back to the car, then selected my AED performance 93 tune and before I uploaded I changed the gear ratio to the new 3.73. Tune loaded fine etc but my speedo is showing 10mph faster than what I'm actually driving, tire size has remained the same what am I missing? Thought the gear selection change in my X4 should calibrate everything?
 

stangn

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My tuner was off as well. Just try the 3:55 gear in the tuner maybe it'll be closer. Otherwise it'll need to be put on a dyno and a programing change made.
 

Big Q

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Interesting, looks like I can play around with revs per mile as well so might try that and see what happens.
 

sbc408

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Get the Revs per mile for your tire size off the manufacturers website and plug that in. As long as the gear ratio is correct you should be GTG.
 

FiveLeeter918

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Useful math problem...

Go into the tune and verify that the rear gear ratio is correct for what you have. Also write down the current tire revs/mile.

Use phone's GPS and get to 60 mph (on your car speedo) on a safe road. Look at GPS to see actual speed.

Needed Revs/mile = (revs/mine in programmer) * (gps speed)/(speedo speed)

You'll find that the number it gives is typically a good deal higher than what you thought.

Reload tune, then go for a drive a verify. If additional changes are needed, you might need to rerun the problem.
 

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