88,
Like someone else posted, I'd check that you don't have your magnet going through the dead spot/center of the pickup or maybe you have a bad pickup. My mileage readings are consistent once the wheel is rotating at a decent RPM for a long enough sample (usually only a couple seconds).
Barrie,
The way I look at it, all the speedometer/odometer is doing is counting RPM and then multiplying by the the number off mm we enter as the effective circumference of the tire and then doing the math to convert the units to miles/km. The most exact number would be to use a GPS to help dial in the number of mm that would give the Trailtech the best approximation. I used the cirumference of the tire gathered by a chalk line or there's always the diameter * PI. Those measurements wont be as exact as what the GPS can help derive because anything dynamic (air in the tire heating up, tire wear over time, etc) that affects the diameter of the tire won't be measured in a static measurement.
Best,
Greg