I can't comment on your Tom Tom, because I don't have one. Mine's a Garmin hiking GPS and it measures both current speed and elapsed distance. I just sat it up on the dashboard and confirmed the speed. The position accuracy is within 10-16 feet (WAAS enabled) and the steady state velocity accuracy is 0.05 meters/sec which is about 0.1 MPH if I converted properly.
Regardless, I suspect your GPS measurement is pretty close too. When I apply the error to your TS figure: 22.8/23.3 X 2575 = 2520, which is VERY close to the original 2522 figure that you started out with, and which I still think is closer to the right value.
Honestly, I'd put in 2522 and call it good enough until you get some data that PROVES otherwise.
Edit: Guess I was slow in giving you an answer. Compare your Tom Tom's speed with the Gryphon's speed.
- Jack
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