As it turns out, the GS has a fairly accurate speedometer. On other motorcycles, I've experienced significant error, greater than 10% and always in the optomistic direction. The speedometer indicates you are traveling faster that actual.
I've become addicted to knowing exactly how fast I'm going and seeing a digital display. The Sigma Sport line of bicycle speedometers work well on motorcycles. They are easy to calibrate and will register as high as 300 kph or 186 mph. The sample rate is once every wheel rotation.
On my R1200GS, the installation was rather simple. I mounted the speedometer head near the instrument cluster and attached the wiring to the existing ABS and brake line that runs down the fork assembly. The magnetic reed switch was easy to attach using some small pieces of aluminum and using the same bolt hole that holds the ABS sensor. The most difficult part of the entire installation is lengthing the small wires. I used two very small rare earth magnets from RadioShack attached to the disk rotor and epoxied just to be sure. Total cost is $22 USD.
To calibrate the Sigma Sport, the actual circumference of the front wheel needs to be known. I put a mark on my front tire and rolled it for two complete revolutions, then divided this number by 2. The tire pressure was carefully set prior to making this measurement.
Accuracy in 2.8%, calculated by taking the indicated error (2 mph) and dividing by the actual speed (70 mph). The factory speedometer is run off a speed sensor connected to the rear wheel, so now I am sensing in two separate places.
Several models of Sigma Sport speedometers are available, I usually chose the cheapest model. That provides me with more functionality than I need. Now I have more odometers, total and trip, that can be switched between miles or kilometers simply by pressing a button. The only downside is this is not lighted for night driving. Not a problem for me, I avoid night driving anyway.
It's nice knowing the factory speedometer is rather close. The odometers agree completely but in the U.S. there are legal requirements concerning the accuracy of odometers - part of truth in advertising stuff.
