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Speedometer Error

  • <i class="fa--xf fal fa-check "><svg xmlns="http://www.w3.org/2000/svg" role="img" aria-hidden="true" ><use href="/data/local/icons/light.svg?v=1758297946#check"></use></svg></i> Discussion starter Discussion starter bdalameda
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bdalameda

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Joined
Oct 9, 2025
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56
Location
Monterey, California
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R18B
After mounting my Garmin XT2 on my bike I have noticed that the factory speedometer and digital readout is quite a bit off. It reads about 3 mph fast below 55 and reads 4-5 mph fast at 60 mph and above. Quite a bit off as compared to the Garmin GPS speed at a straight and steady speed which has always been spot on. My Honda also read 3-4 mph fast and it was common knowledge that the ST Hondas were off. Has anyone else noticed the error?
 
I have never used an external monitor to compare speeds and you may be right, however, one thing to bear in mind is that although a GPS device is very reliable, one needs to account for data fluctuations given that the GPS works on the basis of triangulation. By this I mean that the data you get from satellite 1 may not be the same as satellite 2 or 3 and your GPS device will show as data the average of the 3 as a single piece of data, i.e, 55mph when there may have been 54.5, 55 and 55.5.
 
A deviation of 3-5% is the norm and totally intentional; specified by the manufacturers as such ´error´ is a misnomer as it does what it is specced to do ;)

In areas/countries with lots of speed traps it can be perceived as a safety margin 🫣
 
Yeah my speedo reads about 6 km higher than actual. All bikes seem to do it these days. I may try a a taller profile on the rear tire to make that better when the time comes for a tire change. Will raise the rear a bit as well.

Currently I have a 180/65-16 so it'd be a 180/70-16 if they even make that size.
 
Autos are the same except its 1 mph fast on every car.

Just read speedo and a wheel sensor from the scanner.

I will do that, ride my bike and read speed sensors and compare to Speed O Meter

David
 
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