Originally Posted by
twobikes
A couple of weeks ago I took this GPS unit with me in my car and tested it against my car's odometer over about seven miles. The GPS sat on the dash with a lot of glass around it and still gave a reading about 8 percent low.
That sounds about right as car odometers are often a bit optimistic when displaying velocity/distance
If you really want to measure the distance, calibrate a bicycle computer and ride the distance or get a paper map with a ruler or use a computer map that measures distance.