Mr. Silver, your results absolutely make sense. It's all on the algorithms, and how frequently the device is recording and a number of other whizbang mathematical stuff that confuses me. You'll see the same problem when comparing to many websites that are just using GPS data. So read onward . . . .

cunninghamair, barometric altimeters must be used to get readings at all accurate. The Forerunner 305 does not have one. Do some searches on this forum and over on bikejournal for more discussion.

I've used Polar and Ciclo which also have barometric altimeters with very consistent results with my Edge 305 run through GTC and now SportTracks. So, I'm comfortable using those numbers. Accuracy is in the eye of the beholder, it's pretty unanimous that a barometric altimeter is required to get close. Just remember that weather patterns can cause some inconsistency, but generally not as much as GPS-only data.