My E90 OBC was coded to show the digital speed beside the speedometer. It has 2 configurations, to show the speed exactly same with speedometer OR 'true' speed (should be rated from GPS). I noticed the 'TRUE' speed of my car always lower than speedometer about few kmh, eg. speedometer show 115kmh, the digital speed shows 110kmh. All the wheel & tyre are stock, wondering why the reading is quite diff. Any bro did compare/calibrate our speedometer?