I would guess that unless you went through some long areas where there was no coverage, the GPS should be far more accurate than your cyclometer. GPS's are extremely accurate for speed and distance.

Even with hills. I am thinking that the gps measures the distance flat: If I ride a hill I am on the hypotnuse of a triangle and therefore am riding longer than the gps is recording. I did go under a significang number of overpasses so I can see where I might lose some of that distance but this was quite a bit: Actually as I look at my watch it says 74.3 miles: That seems like a lot. I maybe need to get a course that has mile markers and test it. I am just wondering what to believe.

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Jon Bergmann
http://jonbergmann.com

I noticed similar discrepancies on hilly courses (2 of them being official USCF and Track and Field USA courses).

That said, if the GPA is a real GPS, it should not matter whether flat or not as it takes 3D coordinates.

in any case, it's not the first time I have heard this comment.

Doing the math on your hypotenuse hypothesis, you would have to have climbed and descended 29.7 miles over your 74.3 mile ride to have the hypotenuse equal 80 miles. Besides, if this was the problem, the GPS would be reading less than the watch, not more, so I don't think that this is the problem. Doing a little more math, the difference between the two readings is about 7%. I don't mean to insult you with this simplistic question, but are you sure you've set the correct wheel circumference in your bike computer? This alone could account for a 7% error. Sorry to be the math nerd on a Monday morning- too much coffee.[reply][/reply]

Does anyone know if they (Timex) will come out with a GPS and heart rate monitor system together? Want to get the GPS but hate the thought of wearing 2 watches.

Yes they are releasing the system this spring. It's called the Bodylink system.

http://www.bodylink.info/

Good point: I did more math and realized that I would have had to have an average incline or decline of 21 percent grade. Now that would be something. I will recheck the calibration of my bike computer. I also wonder if it has something with the fact that I purchased a wireless bike computer and it seems skitish at best.

Another variable that makes me courious is that there were a few times when my gps watch beeped at me and I think I had lost the signal. So it could be that the bike computer is off a bit and the gps lost the signal for a time. What happens if the gps loses a signal and then picks up the signal again. If the route is curvy does the gps assume that you have gone in a straight line when it finds you again? Just some thoughts.

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Jon Bergmann
http://jonbergmann.com

7% is the difference between 74.3 and 80 miles.

It is also the difference between 650 and 700 wheels.

If you are riding 650's I would check the cycle computer settings...

I do have 650 wheels and recently I replaced the battery in the bike computer: You are probably right.

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Jon Bergmann
http://jonbergmann.com

I'd think the Timex BodyLink has great potential... it's a radical way to not build in obsolesence. If they make it as good as the Timex S+D, it has the potential of being THE benchmark wireless HRM and Speed & Distance device.

FWIW Joe Moya

If it's only lightly rolling hills and an out an back course... then, my best guess is that the GPS is more accurate. However, if the hills are numerous and steep and your doing a circle course... then, the GPS may be incorrect. GPS only records linear distance... and, it is within a few inches to a few feet (for long courses) of accuracy with no major physical obstructions between satalites and the receiver.

I've used the Timex S+D for about a year... it's very accurate where I live. So accurate in fact, it seems uncanny.

FWIW Joe Moya

The course was in and through denver. Mostly rolling hills with a good hill at the 1/2 way mark (uphill against a strong wind). But then I turned around and had the wind at my back and downhill. I reset my bike computer but have not had a chance to ride it again for comparison. I still have a job and a life--though my friends might not say that when I go for 70+ mile bike rides.

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Jon Bergmann
http://jonbergmann.com