There's no mystery about speedometer error when changing from the factory 14-tooth to a 13-tooth front sprocket. The error is EXACTLY (13/14*100)-100 = -7.14%. By fitting a SpeedoDRD and entering the number -7.1 from the above equation, (must be rounded to 1 digit after the decimal point), the error is eliminated precisely. GPS checking has proved to my satisfaction that the very tiny error remaining is due only to tire wear (or lack of it!) and road surface. BTW, the 13-tooth front sprocket is perfect for the hilly roads on which I ride.