Rad Rhino/Rover 6 speedometer issue

[Full Throttle]

Good day dear fellow RADicals!

For the past 3 weeks I have been the proud owner of a RadRhino 6+ and I am very pleased with it.
Last sunday I went out riding with my brother who owns the model 5 and it occured to me his speedometer differs by 3 kmph.
My 30kmph is his 27kmph. (Sorry to bother you all with the metric stuff xD )
Compared to GPS the difference is even bigger, about 6 or 7 kmph.

I suspect its the common 26" wheel size default setting thing, HOWEVER it seems there is no such setting in the RadRhino6+/RadRover6+
I can confirm the hidden speed menu still works but it only allows changing that, the max speed.
(In my case max 32 kmph... which is fine, I'm not interested in going much faster)

Its not a huge deal, but it annoys me to know its not accurate, im a bit of a numbercruncher and a datacollector, I need precise data to survive  , so if any of you have any ideas on the matter OR know where to set the correct wheel size you'd make me a very happy RADical!