I have a forerunner 305 and never had an issue at all. It is close to my bike computer which is dead accurate. I have done century's and almost do difference. THe last century my bike hit 100 and the garmin was 99.94. The top speed on the bike computer is a bit faster all the time the garmin requires you to keep the speed for a certain distance. Typically I hit top speeds on a ride around 31-35mph and the garmin will show about be 1/2 of a mph slower. Today on my 54 mile ride top speed on the bike was 33.1 and the garmin was 32.8. Do the roll out measurement for your bike computer and it will be dead accurate the garmin for pratical purposes it just as accurate.