HELP!

am having problems re-calibrating speedometer on my 08 GT.
Tires were changed from 17 to 18's. when re-calibrated, speedometer still off,
checked with another vehicle and GPS, when running 70 in mercury, GT shows 90 on speedometer.

if gear box has been changed to either 3.73 or 3.55, would this account for difference.

thanks for any help figuring this out. what's the cure.

Runner

 

Yes, changing gears will affect that. I've always used a tuner to set the gear ratio and tire revs per mile. I'm not sure how you'd do it without the tuner.

 

Rim size makes no difference in speedo....it is the tire diameter which can be the same for various size wheels. Also, yes, the rear end gearing will change things too.