If I go up a big hill does my GPS give a distance based on the horizontal distance between the top and bottom or does it calculate it on altitude too?
Taking it to an extreme if I climbed a vertical wall 1 mile high will the GPS tell me I've travelled 1 mile or 0 miles.
What I'm getting at is; am I actually faster up hills than my GPS is telling me, in which case I'm fitter than I think and don't need to feel bad about drinking lots of beer tonight?
Horizontal distance
Most GPSs will tell you your rate of altitude gain/loss. Not sure if that's of any use to you...
That statistic was quite interesting when I had my GPS running for take-off and landing in a jetliner. (The plane didn't crash, BTW, despite their warnings to turn electronic stuff off.)
Either way, unfortunately the difference is absolutely tiny (unless you are actually ascending vertical walls for kilometres on end); if you do a 30km loop with an even 1km climb / 1km descent you've only gone an extra 67m in total.