Hont said:
Wouldn't GPS have the same issue though? I noticed when I switched from a comp that measured my wheel revolutions to a GPS, the GPS had me going half a mile less on a 15 mile run.
That's about a 3.5% discrepancy - was the 15 miles repeatedly up and down a 25% hill or something!
GPS odometers should be pretty accurate though they obviously only sample your position at intervals so if you were on a very twisty turny route some of your meanderings might get smoothed out thus reducing the distance measured.
You might be able to change the GPS sampling rate. I've just checked my Garmin and its tracklog sampling rate can be set to any one of 5 values (more frequent sampling is more accurate but uses up the tracklog memory more quickly). The sampling can be done at regular distance intervals, regular time intervals or done automatically by the GPS unit whenever it calculates it to be necessary.
I think that is more likely that your cycle computer probably wasn't calibrated accurately. Tyres squash down a little when you sit on your bike thus effectively reducing the diameter of your wheels. That means they turn round more times in a given distance than if you push the bike along without you sitting on it. Ideally, you'd calibrate the computer by measuring the effective diameter of your wheel with your tyres pumped to their normal pressures and you sat on the bike. I bet most people don't do that.
The hilly part of my ride on Sunday was effectively the equivalent of riding about 40 km up a mountain 2,250 m high. Using Pythagoras' Theorem, the distance travelled along the hypotenuse (road) would actually be 40.06 km rather than 40.00 km - only 60 metres difference in 40,000 metres. As I said, for any sane ride in the UK - you can forget about the error.
If you could find a very steep hill long enough, then things would be different. 40km along a road at a nominal 25% gradient would produce an error of more than 1.2 km compared to the value read off a map.