Passed one of those radar trailers which show your speed today. I always suspected that the speedometer was reading too fast, and the radar indicated speed which was 7 MPH slower than the speedometer. Bad news is the bad reading - Good news is - I'm driving too slow. Any clues on adjustment/calibration of the unit? I have to figure that if I am reading too fast, that I might also be registering extra mileage - not a good thing.