I guess I'll have to do a search to see if anyone else has check, but why not take a drive? I used OBD Fusion over CarPlay and the Instrument Cluster to compare displayed, ecm, and gps speeds. I used OBD Fusion because Apple Maps does not display speed and I didn't want to install the Google Maps app. At maybe 40 mph, it seems the instrument cluster display is, maybe a little less than 2 mph higher than ecm (wheel speed sensors?). GPS, interestingly is a little closer to the displayed speed. What is it at 60 or 70? I'll probably see later. I also displayed engine rpm, coolant temperature, oil temperature, and battery soc. While I was focused on speeds, and of course the road, lol, the other stuff looked interesting. Engine seems to hang out at 1300 rpm, and oil temp seems to lag coolant temp, didn't really look at soc because it was off the screen, lol. It'll probably be interesting for a couple weeks or so, lol. note: the screens are different because I figured out I had to give location permission to OBD Fusion to get gps speed to work, duh.
When your using GPS as your point of reference, you can't be going up and down hills or tree lined highways for best results. Hills shorten distance traveled and tree causes intermittent satellite communications. A long flat straightaway should result in your most accurate readings.
Speedometers have been biased high for generations, as an industry norm. The bias was higher back in the old analog days, when meters had greater errors but still had to conform to local laws forbidding them from reading lower than true speed. Euro-market cars historically had even greater high biases than North-American-market cars. As you found, this intentional bias is not necessarily present in the car's internal computers, only in the display to the driver. This doesn't mean odometers have the same bias. In fact, my most recent cars have odometers biased low, following a series of class action lawsuits over alleged warranty fraud for odometers that ran out the warranty too quickly.
