The gear failed in the 91, it's notoriously common. ...
...It's unlikely anyone would go to that extreme for odometer fraud considering they could just roll back the numbers at that stage.
1991 5.0, As I said, you have to take it out to disable just the odo.
But as was said, once it's out, it's easy.
I wrote my numbers down just to make sure I didn't change them
; shops report mileage these days, so a change shows up if you look far enough.
We have a how-to:
Note: It's a Felony in most states to change an odometer reading without reporting it. No guns, no voting. Roommate named Bubba... Who really doesn't like (your kind), lol.
If you think You're a victim of fraud, I'd search for vehicle history records for that vin.
Mileage is recorded at several places; oil change places, state inspections, dealers, etc.
Check it out.
You probably just have a broken gear.
To judge the wear on the car is tougher.
The best wear indicator I've found is the engines, and they can be easily changed.
But, there's a date sticker on the valvecover, with the build date on it, if it's original. But the original VC could be swapped.
The Cam wear is the best indicator I've found, for verifying mileage claims.
A minimally worn cam is worn (polished, I would call it) only in the middle, and by ~130k miles, it spreads across the whole cam surface. Experience: 1 new (dull finish), one 725 miles, one 100k-133k miles (vc off multi times), one 158k miles.)
IDK if you can get a pic thru the oil fill of a cam lobe, but if you can, I'll try to estimate it's mileage.
These wear pretty slowly, but more at lower miles; I think the rollers slide some when their new.