Quote:
Originally Posted by Freetime
Or skip all the calculations and put a cellphone with a GPS speedometer in your car and test it. I'm stock and I tested mine the first day I got it. 80mph on the dash and on the phone. Spot on.
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Came to post this, both my dash GPS in my other car and my cellphone have it. The one on my cell measures down to 1/10ths of a MPH, although to eliminate any risk of error it's best to do it on a straight and level road. Some do not compensate perfectly for altitude changes despite displaying both elevation and speed. Elevation will always be correct, but GPS usually only measures ground speed, not actual speed.