> Considering the cyclometer takes distance based on every rotation of the wheel, while 655 of 2235 trackpoints on the eTrex were over 100 meters away from their previous trackpoint, I'd say the cyclometer was probably spot on for the distance of wheel revolutions. Good lord knows I don't ride any 100 meter stretch in a perfectly straight line - and some of the track points were 270 meters apart. <

Thanks for the info.

I never had a GPS, but I wonder about somethings. You mention the track points which I assume are the specific points a new reading is made?

If so, then I assume that the distance is based on a straight line between those two track points.

If you are going around curves, this would obviously give the wrong distance and if you are doing sharp turns like switchbacks on a mountain, it would be way off. If changing altititude, that would give erroneous figures too, but I think it takes a pretty steep hill to really make a notable difference.

But if you were riding a straight and level road (granted, not likely for a century!), then it would not matter how often the reading were made, would it? The only factor creating error would be the built in error. So, if the built-in error was plus or minus 50 ft, then the maximum error would be 100 feet (if one end of the ride was in error by +50 and the other end was off by -50).

Now, if you rode a straight course for 10 miles (52,800 feet), you would be off no more than 2/10 of 1 percent! If it was just 5 miles, the error could be 4/10 of 1 percent. Heck, if you can get even a two mile straight course, your would still be within 1 percent. Doing the test periodically and averaging the distances would increase accuracy considerably.

I don't know how accurate your GPS or other ones actually are, but I thought 50 feet was likely a worse case scenario for a decent unit.

Granted, none of us holds a perfectly straight line when riding, but the GPS info would help narrow down the error. I mean, if the GPS is, at worst, off by 100 feet on either the high or low end, then you know, wobbles or not, you can't have a result lower than the lowest possible GPS result. If you do, then you calibrated too low. The high end is a bit trickier since you can't tell how much you will wobble from a straight line.

Bob