Joe, if you want to check to see if your speedometer is right on, find a stretch of hiway that is flat and straight. Hold your speed dead on at 60 MPH and time it between mile markers. If it takes 60 seconds, you're right on! If it takes less time, then you are running faster than what the speedometer shows, and if it takes longer, then you are running slower than what your speedometer shows. If the time is not off any more than 5 seconds from 60 seconds (55 thru 59 or 61 thru 65 secs.) then you can figure about one mile per hour per second.