I was gifted a Garmin Forerunner 305 (heck yes) a few months ago and am still getting used to using it. I'm wondering if I'm doing something wrong, because it keeps coming up a little short, especially on the bike. I did the MS ride last weekend, and was much closer to 55 miles than the supposed 62 on the cue sheet. This weekend I did a half marathon out in Kentucky (which I'm also going to post separately about), and on a bunch of the supposed mile markers the Garmin was behind, and came up around 12.95 instead of 13.1. Does anyone else have this problem?
GPS units sometimes come up short because they sample position at discrete intervals, so they cut corners and think you ran the diagonal straight between a point before and a point after a sharp turn. That said, 7 miles out of 62 seems a bit much. That run accuracy was about 1%, not too bad for a consumer item.
I don't have one, but I doubt there is anything you can do wrong to affect the accuracy.