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I just installed a Specialized Speed Zone Sport Classic cyclocomputer. After calibrating it using the tape measure method, I tested it against a Magellan Sport Track Pro GPS.

When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.

The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.
 

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The GPS is probably off for the reasons you have stated, but the cyclocomputer is off as well. When you are making sharp turn, you are traveling on the sides of the tires which is a smaller diameter than the one inputted to the computer. Having a smaller diameter than was inputted will lead to longer distances and faster speeds.


billee said:
I just installed a Specialized Speed Zone Sport Classic cyclocomputer. After calibrating it using the tape measure method, I tested it against a Magellan Sport Track Pro GPS.

When running straight, the speed readings and odometers matched exactly. While making sharp turns, the GPS speed would read lower and after 10 times around a one mile circuit (with six sharp turns) the GPS odometer read 0.3 miles less than the cyclocomputer. This error appears to be due to the one second sampling rate of the GPS. During a turn the GPS is calculating the straight line distance traveled each second rather than the distance along the arc being traveled. In the extreme case, if you were making a complete circle every second, the GPS would calculate a speed and distance of zero.

The error is probably not significant under most conditions but if you need to know accurate distance or average speed over a course with a lot of sharp turns, use a cyclocomputer.
 

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GPS more accurate

An engineer friend of mine that's familiar with the operation of GPS has the potential to be more accurate if it's implemented correctly. Utilizing satellite triangulation is the most accurate because elevation can be included in the calculations. Typical altimeters use barametric pressure which can vary and cause error depending on the weather.
 

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Yeah, in a straight line, like driving down the freeway, your GPS is more accurate than even your car's speedo and odo.

The reason for the "shortening" on a twisty mtb ride is that the GPS "draws" straight lines from each "one second sampling point", rather than that curve you actually rode. If you take a switchback, for example, your tires might roll 3 or 4 revolutions going around the curve, but the GPS might hit samples at the entrance and exit of the curve, which might be only 3 feet apart when it "draws" the straight line between the points, cutting off the curve.

The altimeter on your GPS is very accurate...often much better than a barometric unit.
 
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