Took the car to have the speedo checked. It's 7% fast: when indicating 100 km/h, actual speed is 93.
When the Scangauge is showing 100 km/h, both my Garmin GPS and Waze app are showing 101 km/h. I'll be keeping KM/H on the Scangauge display.
I find that level of inaccuracy unacceptable.