Re: Speedometer Accuracy

[Marco S Hyman <marc@xxxxxxxxx>]

"Pete and Nat Lavengood" writes:
 > I also use a GPS and the bicycle speedo and they agree and I believe they 
 > are accurate.  Pete

But the question is how accurate?  My old K12 had an accurate speedo, if
your measure of accuracy was +/- 10% :-)  The typical GPS has a precision of
0.1 MPH, but its accuracy is more in line of +/- 0.5 MPH.

The thing to remember is that the accuracy of a GPS is NOT a constant.
A partially blocked view of the sky effects accuracy.   So does the DOP.
See http://www.cmtinc.com/gpsbook/chap7.html for DOP info.

Things got much better when SA (Selective Availability) got turned off.
As I mentioned in an earlier posting... before SA was turned off my garmin
gps III and gps III+ would both sometimes read 1-2 MPH... when standing
still.   That kind of tells me that the speedo function accuracy of those
units was no better than 2 MPH.

// marc

------------------------------