Originally Posted by
bikerjp
I believe both the 305 and 500 have a barometric altimeter which should be similarly accurate. If the 500 doesn't produce the same data as the 305 (take both on the same trip if you haven't) it could be due to improvements in the newer model but that's just speculation. I have noted that my 500 gives fairly consistent elevation data over repeated rides on the same courses.
My downloaded data is pretty close; it's the info while riding that is suspect, especially elevations. Even the speed jumps around occasionally while riding through my flat corn country with unobstructed reception.
I just got it so I might check the firmware and try some of the other suggestions.