think our mileage was rolled back

[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.