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BMW speedo's are notoriously about 8-10% optimistic.
A GPS is accurate on flat, straight pavement, BUT, it drops off significantly in curves and where there are large elevation changes. It also depends on time of day and date. Why? Because the accuracy of any GPS reading depends on the configuration of the satellites. Those configurations change moment by moment. It is basic solid geometry. Before surveyors go out to take GPS readings they check for the best configuration times. I believe that nifty piece of software you speak of is one of two I am familiar with: TuneBoy or TuneECU. They program the ECU directly which GS911 does not do. There is a box called Yellow Box that calibrates MC speedo's and another called something like Speedo Calibrator.The GPS is far more accurate than the speedometer. On a level road, it's as accurate as anyone would need it to be. My RT had a small difference between indicated and actual, my GS/A is 6mph optimistic at 70mph. There's a nifty bit of software that can correct this on other bikes, I'm surprised the GS-911 can't do it for ours.
There is a difference of approx. 5mpg between my Zumo 665 and the speedometer. The GPS has me going slower than the speedometer shows. So which do you think is more accurate? I assume it is the GPS, but I don't wish to bet a speeding ticket on a hunch.
ALL motorcycle manufacturers degrade the accuracy of their speedometers by 3-7%, making them read faster than your actual groundspeed. This avoids what could have been a deluge of accident litagation.
The GPS is the more accurate.
On my work truck the speedometer shows 2 MPH slower than the GPS.
Don't own a modern beemer, do you?
My hexhead speedo reads about 1 1/5 MPH faster than a GPS at all speeds I travel. According to reports I've read most hexheads read 1-2 MPH fast.
A GPS is accurate on flat, straight pavement, BUT, it drops off significantly in curves and where there are large elevation changes. It also depends on time of day and date. Why? Because the accuracy of any GPS reading depends on the configuration of the satellites. Those configurations change moment by moment. It is basic solid geometry. Before surveyors go out to take GPS readings they check for the best configuration times. I believe that nifty piece of software you speak of is one of two I am familiar with: TuneBoy or TuneECU. They program the ECU directly which GS911 does not do. There is a box called Yellow Box that calibrates MC speedo's and another called something like Speedo Calibrator.