How well the satellite is picking you up also impacts your speed reading. Yesterday we were riding on a very wooded trail. DH's computer is wired and said 18 mph. Garmin said 12 mph under the trees. Then we'd break out of the trees into a sunny spot for a few minutes and Garmin would read 18 mph. Then back under the trees and the speed would drop. I didn't realize it was that touchy.