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Discussion Starter #1
Has anyone done any comprehensive verification of the speedometer accuracy on their M5?

I used a handheld GPS to check mine in 10mph increments up to 90mph today. The speedo was more accurate than I expected. I was prepared to find out that it was 5-10% optimistic at highway speeds, but it was dead accurate up to 70mph and then it started getting slighly optimistic. 90mph on the dial regestered 88.3mph on the GPS. That's less than 2% off.

Does anyone know if this is linear or does it get progressively more optimistic at higher speeds?

Rick W
 

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It gets optimistic. Last night, my speedo registered 165 - 170 mph, which should be impossible if the 155 mph limiter kicked in. Of course, I wouldn't think of doing such high speed runs on a public road.

John Hennessey wrote this on his website:

Top Speed - the stock M5 will attain a true top speed of 159 mph while the speedometer indicates 166 mph. The factory speed limiter is a soft limiter and cuts back power just enough to maintain top speed without exceeding it. We have measured a speedometer error of 3-4% from 80 - 159 mph. These results were verified with both the M5 avg speed computer and our Stalker radar gun.
 

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rick,
for what it is worth (sometimes seen as "fwiw"), there is about a 3% discrepancy between my speedometer and on-board computer. this doesn't seem to vary by speed. oh, and the speedometer reads higher than the obd. thanks for your results.
michael
 
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