I was driving my son's 2006 Mini S from the Woodlands, TX to Galveston County last night and had the cruise set on 65 MPH. I noticed everyone was passing me so I looked at the speed on the GPS and it said 61 MPH. The speed on the display on the tach and the main speedometer both said 65 MPH. The tires are factory size with the factory rims. How can I correct the speed to become accurate?