Yesterday I was finally able to install my GPS, so today we went on a 108 mile ride to check the speedometer. I have a 2006 B400. Here is what I found. Up to 40 mph the difference is about 2 to 3 mph, with the GPS showing me going slower than my speedometer said I was. The difference increased slowly as the speed of the bike increased, so that at 40 mph indicated on the bike my GPS showed 36. By the time I reached 65 on the bike speedometer, the error was more apparent in that it was now 5 mph off. It stayed that way up to 70 where I decided not to go past. So the error is NOT 10%, which is what I have heard they were off, but still it would be nice to know what our REAL speed was. Well, at least it is in favor of less rather than more because I wouldn't want to try to fight a ticket where my speedometer was showing me going slower than I really was going. Here is a picture of my GPS setup, for those interested.
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