Huh. I set the home elevation on my 800. I use the Garmin speed/cadence sensor. My Garmin is out in the ambient for at least an hour before I start riding. Don't know what makes the difference, but my Garmin data is alwayis spot on. When I ride a loop, ascent and descent are almost exactly the same, and the numbers are in good agreement with the RWGPS numbers for that route. Last 44 mile ride I did, ascent and descent differed by 3'. Unless something really weird is going on with your weather, barometric elevation shouldn't vary by more than 50' during a normal ride. Either you're doing it wrong or you have a defective unit.