According to a QLD police Radar gun. My M3 is traveling at 136km/h when cruise control is set to 140.
Don't ask how I know this.
In general. All car speedos read a couple % higher than your actual speed.
2012 AW e90 M3 DCT - Daily
2010 AW X6 50i - Tow rig/baby hauler
1991 Nissan 180sx SR20DET - Drift pig