I thought maybe my GPS unit gives faulty readings in miles, but might possibly give a correct reading in kilometers. It was made in a country that uses the metric system. Perhaps they got the software wrong for expressing in miles. But, alas, my normal ride should register as 23 km. My unit showed 21.2 km. The error is right at 8%.

During my ride today I noted the reading between two points that show as one mile exactly on www.bikely.com. My speedometer showed one mile at the end of that stretch. My GPS showed less than the equivalent of a mile (about 1.3 km, if I remember correctly). I glanced periodically at the symbol indicating satellite acquisitiion. It indicated I had satellites every time I looked at it.

An 8% error is not too bad, if the unit is consistent. I can factor to find my true distance covered. But, there have been times when readings have not been consistent.