727 speedometer gear

68 Coronet RT said
I have been having the same issue with my speedo showing faster than my GPS. I read your post about measuring the tire size as I'm running the same tire as the OP 245/60/15's. I'm running 32psi in the tires and I took a measurement to the center of the wheel X2 and I come up with 25". So the chart shows I should run a 40 gear which I had in there and she read 5 mph fast, so should I go to a 42 tooth gear ? I went to a 38 as someone suggested and now I'm 10 mph fast on the speedometer to my GPS.
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If your speedometer reads "65" and the GPS says "60", then you're reading 8.3% high (65/60=1.083). So 1.083 x 40 tooth = 43 tooth. The 43 tooth will probably be the closest. I just re-read AJ/FormS's post above, and we are both using the same logic. This is the most accurate way to "calibrate" your speedometer. Basically trial and error.