15's are stock in the 1989 model year. The switch to 16's didn't happen untill 90' i believe. I have drove an AOD equiped car before, and the 4th, or OD, usually kicks in really low, like around 40 to 45 MPH or so. But on the highway you should see about 2k or so in OD. I'm almost certain that the dealer didn't check to see if the rearend gears have been swapped out. Factory gears in a auto-equipped car were usually 3.27's, Although i have seen some with 2.73's. There are 4 ways to see what gear you have.
1. Jack the car up and manually count the driveshaft/tire revolutions.
1. Jack up and support the rear of the vehicle.
2. put the transmission in neutral, and disenguage the e-brake.
3. grab a friend, you will need two people to do it accurately this way.
4. mark the driveshaft back by where it bolts to the differential, just draw a short line on the driveshaft, going straight with the driveshaft.
5. Mark the tire, preferrably at the 12, 3, 6, or 9 o'clock position.
6. tell your friend to SLOWLY rotate the tire forward or backward untill they turn it one complete revolution, as they are doing this count how many times the driveshaft turns.
7. make sure both back wheels are turning the same direction, if the other tire spins the opposite way you friend is spinning his, discontinue this test, it will be inaccurate. You have a defective limited slip clutch packs in your rearend or you have an open differential and you will have to determine the ratio by running the tag on the differential or the one on the driver side door.
8. write down how many times the driveshaft turns.
9. say your driveshaft turned 3 and roughly 1/4 turns, turn that into a decimal, 3.25. since 3.25 isn't a ratio that is offered, you most likely have 3.27's, a common gear.
2. Crawl underneath the car and write down all the numbers on the differential tag.
1. get under the car and write down the numbers on the differential tag, it's held on to the differential by one of the cover bolts.
2. do a search on stangnet, or the internet in general about decodeing the numbers.
3. Run the axle code on the options sticker located on the driver side door sill.
1. Open up your driver side door, on the options tag there will be a box that has the letters AX on it. This is your axle code.
2. do a search on stangnet for the decodeing information.
4. Pull the differential cover off and count the teeth on each gear.
1. jack up and support the car.
2. pull the differential cover off.
3. count the number of teeth on the ring (big) gear.
4. count the number of teeth on the pinion (small) gear.
5. divide the number of teeth on the ring gear by the number of teeth on the small gear.
two ideas can potentially be misleading though, what if the door has been changed? What if the axle tag is missing?
The only sure way is to jack up the car and count it manually, or remove the cover and count manually.
My friggin fingers are tired from typing now......