Hi,
It's very easy to get a pretty accurate calibration of both odometer and speedometer by driving a few miles on the interstate. The mile markers posted on the side of the road are usually quite accurate in their placement, so you can calibrate the odometer by recording the odometer reading at a milepost, then driving 10 or so miles by the posts numbers and recheck the odometer reading. From this you can calculate the percentage of error. Speedometer calibration involves driving at a constant speed for a mile or two and recording the time between mileposts with a stopwatch. At 60 mph it takes 60 seconds to drive a mile, so divide 3600 (seconds per hour) by the number of seconds it takes to drive a mile and the result is your true speed in mph for any one indicated speed. Accuracy will improve if you check the time for 2 or 3 miles.
Bob N.
