I consider 1% way off! 3% is not even in the ballpark. My bike computers are at ~0.3% (compared to true distances from a digital map released by our national mapping service) at worst, and usually better. And even then, you're getting good numbers for a GPS. Last time I walked through a built-up area with a GPS unit, it regularly "teleported" me 200-300 ft in a random direction, simply because GPS doesn't work well when you don't have an open sky around you.