As it happens, today I was out trying my new phone mount with Waze & notice the difference in speed between Waze GPS & the speedo. When the speedo was at 50kph the GPS said 46, increase speed to 54kh on speedo & GPS says 50kph. Righto I think, just add 4kph & I'll be right on the money. Nope. At 104kph indicated on speedo GPS says 96kph. increase to 108kph on speedo & GPS says 100kph.
The speedo is displaying 8% too high for every speed, did a little mental arithmetic while riding & came up with 54k's indicated is 50, 75k's indicated is 70 & 108k's indicated is 100 for when I don't have the phone /GPS setup functioning or can't see it because of glare.
A couple of other bikes I've had have been the same, but thought a new era bike with digital dash would be a lot more accurate. I had Speedohealers on the others & got them to read spot on, but then the ODO was slow, so ya can't have it all. I took the right speed over the distance. One bike was 7.5% too high & think the other was 11%. My R3GT is 8%, so some things don't change.