From what I've read - not direct knowledge - not all odometers are the same. It's possible you got one for a different axle ratio. I would try to find a measured mile somewhere and do the stopwatch vs. speedometer/odometer thing - i.e., drive thru the measured mile at a constant speed (60 would be convenient) and time the event and note the odometer at start and finish. If there's a disconnect between the time and speed, then you have an uncalibrated speedo/odo which is throwing your mileage readings off.