As the odometer runs directly off of the speedometer cable from the transmisson, the gears in the trans that turn the cable would have to be waaayyy off to accumulate that many extra miles, it seems. Is the rear axle ratio the same as the car was built with? Might the speedometer be calibrated in kilometers rather than miles? Tire size similar to the size the car came with originally?
It would seem that if the odometer is that far off, then the speed indicated should be waaayyyy high.
Might need to verify which rear axle gears are in the car. Then look in the FSM for the speedometer drive/dirven gear combination for that axle and the OEM tire size coverted to current P-metric tire sizes.
IF everything checks out so far, then you'll need to remove the speedometer head and inspect the ocometer itself. The washers between each number have a tang on them which is held in place by a white plastic "panel" that snaps over the tangs. That holds things secure so that as the numbers on the odometer turn, they only do so in the "normal" sequence/manner. If that while plastic panel might have a break in it, the numbers can turn more than they need to and not be horizontally aligned as they should be. A new plastic panel will be needed. An easy fix, BUT it takes a lot of effort to get to the point to inspect/replace it.
Just the R&R of the speedometer head will be covered in the FSM, as the odometer repair/replacement would need to be done by a speedometer shop, for many reasons (at the new car dealership level of things) when the cars were new/newer and odometer readings should be accurate. So, make an accurate assessment of the possible current mileage to reset the odometer to.
Just some thoughts,
CBODY67