Need help from a speedometer gear size expert!

trapster said

4 mph at what speed ? The way I do it is to find a mile marker and note your odometer. drive 10 miles and note your odometer. Divide the difference by 10 and multiply that number by the number of teeth. so if your odo says you drove 9 miles and your tooth count is 30 then you will need a 30 x 9/10 = 27 tooth gear. or just use the charts.

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My very accurate speed app on my cell phone tells me that I'm actually traveling four mph faster than what my speedometer is showing. At 74 mph on my phone, my speedometer reads 70 mph. I've checked the speed app against the very accurate speedometers on my Lexus and my Mercedes, and the app is right on. So I'm pretty sure I'm 4 mph high at 70 mph. I'm thinking about installing a pinion gear with one fewer teeth, which should speed up my speedometer.