My speedo is darn near perfect but my odometer is off by a good 5% which worked out to something like 2 miles a gallon for me. The factory error made me think I was getting better gas mileage than I really was.
I agree that as long as the bike can maintain crusing speed and the RPM showing on the tach. is correct all is well. However when your speedo shows 65mph and the policemans radar says 80mph you have a problem.