Joe is right in saying the odometer test will tell you if the "gearing" for the speedometer is right. Do the "10 mile" test he recommends, but keeping a "steady speed" is not necessary.

Originally Posted by Joe H
Drive ten miles at a steady speed on a marked highway and write down the mileage shown compared to marker signs.

Adjust gears or add adaptor to correct the odometer.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.