There was a discussion here last summer about the accuracy of elevation on a Garmin 305. The consensus was the elevation is inaccurate with this device--I forget why. However, if the numbers looks impressive--I say they are correct

I use a Garmin 305 with MotionBased. I have a Mac and could only use the Garmin software when it was updated recently. I downloaded some old rides and the numbers were slightly different. When I compare distance and speed with two other riders using different brands of bike computers--it is very accurate.

Motionbased does have a very active and informative forum--it was purchased by Garmin.