[Argess]

Required Gear teeth = current teeth x indicated speed / actual speed

i.e. If you have 8 teeth now, 8 x 55/63 = 7

hmmm....I made up that formula in my head, ..... is it right?

Anyway,, don't trust your GPS UNLESS you do a test in both directions. GPS has come a long way, but quite a few years back when I use to test boat speeds, I would take a max speed in both directions due to satellite wander (and wind, current, etc., which isn't applicable to a car) and then settle on the average reading.