You are better off checking the odometer when looking for gear changes. The odometer is gear to gear, speedometer is gear to magnet drive. If the magnet drive is weak, the speedometer can be correct at one speed and off at another, usually the faster you go the farther off it gets.
If you are sure the speedometer is good, use 55 or 65 mph as your target speed if thats where you drive the most. Slower speeds are easier to guest-a-mate.
