think our mileage was rolled back

[1989GT]

Not sure how to tell if the odometer has been rolled or not, but I will say this. I've never seen a ring and pinion need to be replaced from just driving. I have seen them give way in a racing environment. I curious to as why a dealership would but 27k miles on a car. I'd say your best bet would be to take it to a ford dealership and tell them what you suspect and have them look it over. Possible the car's just been driven really hard. Also what gear ratio was in the car? And what ratio did you have put in.
-Ryan

[WHITE04GTVERT]

Sounds like your stang has been modified. The only way to roll back mileage is with the ford dealership diagnostic computer, as the mileage is stored in the computer and even if you change it, it has to be done with there computer or the car will not crank and run until it has been programed with there computer. If your mileage is off by 7 miles for every 30 miles then you have the wrong size tires and the computer has not been recalibrated to the new tire size. you can do this your self by buying a (predator or hypertech type programmer) and connecting it to the diagnostic terminal. If you don't won,t to do this I would take it to a ford dealership and ask them to do it for you. I hope this helps.