Checking Distance

One thing that could be an issue is that even though you have set for the right wheel size, this doesn't take into consideration your particular tire and other related factors. With this in mind I laid out a 100 foot builder's tape measure (cloth tape on a reel), started on one end with valve stem down, and rode along it counting revolutions of the valve stem, noting the feet and inches at the last full revolution before 100 feet. That gave a real life relationship between revolutions and distance over a distance long enough to be fairly accurate, and taking into account any difference that tire, inflation, and rider weight might possibly make. Convert that to the units needed for your computer for circumference (one rev.). More accurate would be an even longer tape, or sequentially laying it out more than one length. If you don't have one of these tapes, use a more available 25 ft tape, and lay it out on a flat road for as many multiples as you have patience for. One limitation is that I had to ride a bit slow to count the revolutions, and this makes for a less straight riding line than at more normal speed. To improve on that, you could have a helper steady you by walking along side, and hopefully keep the track straigher.

Chero