speed measurement (std/cyclo/GPS)

[Frank Boon]

Hi,

Some weeks ago I installed a GPS receiver
onto by S. One of nice things about GPS is
that the speed measurement is not sensitive
to assumptions on wheel size like in the
standard speedometer and the cyclo-things.

I was shocked to see the difference between
the standard speedometer and the GPS unit:

std GPS
80 79 km/h
100 98
120 115
180 162
225 212

It is logical to see the error grow as the
speed increases: a small error in the tire
circumference is multiplied by the RPM of the
wheel. But higher speeds also increase the
cirumference due to centrifugal reaction
forces.

I just thought I should let you know that the
cyclometer unit are also quite inaccurate.


Regards

Frank

PS: These GPS measurements were done with
7 satellites and a normal observation
geometry