Check your speed sensor coming off of the transmission. If your speedo is working, then this is not likely the problem, but the odometer on the car gets its signal from the speed sensor attached to the side of the tranny. I have seen this part go bad before and cause the odometer and speedo not to function correctly. You will also eventually get a "check engine" warning light if this is your problem, but may take several miles before it comes on. Your mileage is stored in the Pollack box control unit that is under the dash. Replacing the actual odometer read out will not change what mileage the car actually shows, so if it is your actual display that is bad and needs to be replaced, it won't change the actual mileage that is displayed.