Ok, Downloaded Waze (which looks like a fantastic App. Google Maps has been my fave App for years, but Waze is right up there).

If only one of them was supported by CarPlay!! (Apple Maps, you suck!).

The speed limit & speedometer features in Waze are great & I just put them to good use around my area, in various speed limit zones.

Here are the Vehicle digital speedo readouts v Waze speed.....

40km/h (Vehicle Speedo): 39km/h (Waze).
50km/h: 48km/h.
60km/h: 58km/h.
80km/h: 77km/h.
100km/h: 97km/h.

To be safe, I won't be worrying about adjusting the speed at below 60km/h, but for 60 & above, I think I can be safe going 2km/h above the vehicle speedo.