Originally Posted by
pdlamb
When I calibrate something, I compare the measurement of the device (the wheel sensor plus whatever's counting wheel revolutions) to a known standard (like a surveyed distance). If I ride a surveyed distance of 5 miles, and my device reads 4.97 miles, its accuracy is -0.6%. 0.6% => magnitude, negative => it measures less than the standard.
Anytime you calibrate a rollout, there is an error associated with that calibration. Later, when you use that rollout to measure distance travelled, there will be a systematic error associated with the rollout calibration and an error associated with the new measurement(s). By nature, you do not know the exact magnitude and sign of these errors – you can only make statistical statements about the errors.
Calibrating your rollout by riding a well known long distance is a good thing, because it can (potentially) reduce the calibration error. It cannot, however, completely eliminate calibration error.
In your example, you've simply determined that the new calibration differs from the previous calibration. The new calibration may or may not be more accurate – it depends on the details of how the two calibrations were made.