Is there any way to adjust the speedo? I'm finding mine to read about 10kph fast at 100kph

 

No , reading high is a UK thing . Must be within 3 % of the truth then they let you go 10% over the limit before you get fined.. Speedos in the US are accurate.
I use a scangauge giving me MPH,eng temp ,bat volts, rpm. sits on top of the wheel binnacle ..had it 20 years . they are now expensive.
Set against my GPS. Or if you have your phone on the dash you can download Radarbot which will alert you to the speeding cameras and give you your speed.

 

I added a Garmin GPS unit to my dash. Get speed, GPS info as needed. Also have a backup camera over the license plate that is activated when I reverse. Works well, not a vision impairment.

 

Is there anyone here with Arduino expertise that would know if our speedos are candidates for such a circuit? My only dealings was with virtual walls for a robot vac and even then, I had to rely on someone that knew C++ programming language.

 

If it is true that the US cars read correctly and the UK cars read 10% high (no idea personally if that is true), then there must be something different between the two. I would guess this is most likely in the Instrument pack firmware, so replacing or reprogramming it somehow to think it is in the US might be a solution, depending on what else it messes up (like having a different definition of gallons, etc.). I assume (maybe wrongly) that the UK version does measure distance correctly, and so only messes with the display for the speedo...?

The ABS module is the one that determines how fast the wheels are going around, and sends that to the other modules over the CANbus. So you could mess with that signal on its way to the INST module (like has been done to spoof the TCM for axle ratio change), but then it probably messes up the odometer....plus it is a significant piece of work to do, so unless it is the most annoying issue in the world I would doubt it is worth the trouble.

 

My 2004 in the UK reads spot on, well within 1%, to GPS.

Maybe the OP (olphart) vehicle has had the wrong size tyres/wheels fitted at some time.