Found out what caused my garmin to report distances incorrectly. When it auto detects the wheel size it then causes the speed and distance to get screwed up.
I can't see how this can happen as it is such a big problem which would be simple to fix. You already have the GPS giving time and distance, so how it can be so wrong is beyond me.
I reset my garmin, then setout on a 50km ride, about 17km in it reported that it had detected the wheel size then my problems started. Finished the ride and it only showed as having ridden 33km. So that's not just a little out! On the remaining 33km it only detected half of those, but the GPS trace is spot on. During this time the speed jumps around from 0 to what I guess would be roughly the correct speed, but most of the time was sitting around 12kmh.
And the HR monitor wouldn't work properly in high winds, throws the readings right up, but reading online this is a common problem.
So far, not overly impressed!
On one of the descents once I reached the bottom, a car passed me, the guy in the passenger seat wound the window down and yelled out 60kmh!!! And the driver punched the air which made me smile
