I made a bracket today so I could mount my Magellan 2200T GPS on my Silverwing. I've been curious to see how much difference there would be between the SW's speedometer and the GPS. I was pleasantly surprised to find the error between them was no more than 3mph up to 70mph. In the 20-50mph range the difference seemed to be around 2.5mph or at least a bit more than 2mph but not quite 3mph. My SW has IRC tires which may produce a different error rate than other brands.

Wow - no one responded to this?? 'sup crew?

Actually I am surprised no one joined in the great intentionally designed faulty OEM Speedometer vs. the all powerful infallible GPS dust up. Of course this may get it going...

Although exavid brought a new variable to the fore - tires.

Yep. Speedometers average 10% over GPS readings.

Some less. Some more. But thats the average.

"Whoomp - there it is!"

I'm not sure about the averaging thing though. And I'll add an additional wrinkle - my wife and I travel together and ride the same route. I invariably accumulate 1.5 to 2.2 miles more on the odometer each tank full. I replace the tires on each bike at the same time and run the same pressures in each set of tires. Granted gravity has more effect on me (mass is greater).

What's that all about?

Had to validate my ol' buddy Dale.

Gravity is an illision perpetrated by skinny people to make big people look bad.
--Jackie Gleason

OK Folks.... knock it off ! ! Just when I get use to following my own 5mph rule (generally above 35 to 40mph I start adding 5mph (+/-) to what the limit sign says) some one starts making percentages or including parts of a mph, etc, etc. Aint had no problems yet but there's that little "thing" lurking in the back of my brain (or is it that "thing" lurking in the back of my little brain) that says I need a better system. Oh well, only time and fate will tell. And yes, I've been too cheap to buy a GPS so far.