I don’t believe wheel size plays any part in that if you have the correct size tyres fitted.

I have 19” wheels on mine and believe the rolling diameter of the wheel is the same as the 17 and 18” rims with the correct tyres fitted.

At 100kph on my GPS the speedo is showing about 103-104 so it’s pretty close.
The tacho shows dead on 2,000 RPM at this speed with the TC fully locked.

I’d be intrigued to know if the trip computer economy calculation can be calibrated.
I know it can on some other cars but not sure about the X5.