F650 GS dakar odmeter gaining 2km on 10km

[Janneman77]

I checked when I ride, my odometer reads 2km more when I do 10km it clocks 12km.
I wonder if this is a common deficit.
As far as I know the rear wheel size can affect this but I bought the bike with this rim so what is the size supposed to be to get the proper reading?
I have a Mitas 140/80B17M/C 69T E-10 TL on the back

 

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