On the next test drive, watch the odometer, it is gear driven right off the cable. The speedometer is magnetic driven and it's accuracy is determined by the magnets strength. If the odometer is correct, then the speedometer head is dirty or weak and its something you will have to clean or live with. Now if the odometer is off, then you can adjust it with gears in the transmission.
Drive at least ten miles to get a good measurement and compare it to your electronic devise or mile markers on the road. Figure the percentage its off and change the gear ratio the same amount. Slow the driven gear down to reduce the milage, or speed it up to increase the miles. The speedometer will speed up or slow down the same percentage.

Joe

1937 Chevrolet Inline-powered Streetrod