Quote Originally Posted by echidna View Post
If the Garmin uses the same GPS base maps as most popular mapping software (and I don't know this for certain but do believe it to be so), it is overestimating the amount of climbing you're doing by 20-25%.
Many of the Garmins (such as my Edge) also have a barometric altimeter that makes the climbing data much more reliable. (Until you run it through something like Motion based, which I don't, but which does bump the climbing about 20 - 25%.)