GPS Speed Accuracy


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Aug 18, 2002
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There seems to be some controversy locally as to whether the MPH readout on the GPS systems is accurate. Mine (Garmin III+) constantly shows my GS speedo to be about 8% fast, which according to most people seems to be par for the course (groan - no I'm not a bloody golfer!)

Also I've heard that fitting a different speedo hub will cure the accuracy problem ? from what model - I personally wouldn't bother but the ability to do some simple arithmatic on the move seems to be a major concern to some sad folks, comments like "oh it will always bug me" - as if there's nowt more to worry about, ah well....

Any comments on either of the above, the first for me & the second for the mentally challenged!

I find the same rate of error when I use my GPS V in the wifes car so I`m assuming its a lag thing with the GPS
Dave Hall said:
I find the same rate of error when I use my GPS V in the wifes car so I`m assuming its a lag thing with the GPS
Think you'll probably find that despite the fact that the GPS relies on satellites miles up in space and the speedo relies on a cable connected to a hub less than a foot or so away from the speedo - the accuracy competition is won hands-down by the GPS.

Yes there is a time-lag, as there is with a regular speedo but constant speeds are more accuratley displayed by the GPS, unless you've done the hub mod mentioned on other threads.
The Road Traffic Act allows a 10% speedo error when deciding to persecute (I think thats spelled correctly) so my GS & wifes car should be about in the envelope.
I don't get my bike serviced as often as someone with a 10% error....


(2.5% error, R1100R hub)
I know that there are a few holes in the argument, but a GPS knows where it is in space because it can very accurately measure time signals emanating from precisely known coordinates - ie the satellites.

So if it knows accurately what the time is and where it is, there is no reason why the speed calculation shouldn't be highly accurate too.

The irony is that BMW speedos are consistently inaccurate - ie all 11GS speedos read about 10% over. If BMW can build them to this level of consistency, it such a shame that they don't make the more accurate too!

Within your GPS book you should find a speed accuracy stats....I think it's something like .05 of a meter per second.

I'm no expert but I agree with Greg, GPS is based on digital fact. There's no room for manufacturing varibles, which in my book would mean if you've a good signal then you'll be getting a bloody accurate speed reading.
I'd be interested to know if anyone else has observed increased accuracy in the cold weather recently.

I think that the little spring inside the speedo is responsible for making it read lower in colder weather.
R100GS speedo constantly reads 10% over compared to my GPS. So they've (BMW) been doing this for years. Could this be why BMWs have a reputation for being able to cover high mileage? They don't actually do that many miles the speedo just clocks up a lot!! e.g. My 60K on the clock is actually only 54K.
My new Triumph Tiger, however, is exactly the same as the GPS. So it can be done!
speedo accuracy

Hi adrian,
Fit the R1100R speedo hub to cure your 10% error-why? well the bike i'm trading in with Shaun in March will have done 2500 miles LESS than it states - but he's giving me a good deal all the same. I've got the speedo hub fitted at the moment and accuracy is 1%. Speak to David at Rainbow it costs £31 and some coppers and takes minutes to fit

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