You will need to check the metal ID tag on the axles to get the actual ratio. You should have a Ford 8.8" rear on the 86 with either a 3.08:1 or 3.55:1 gear ratio. If you can't tell what you have, post the numbers from either the front or rear tag, as they will be similar if not exactly the same. I have not seen a 3.73 0r 4.10 with the 300-I6 motor, but it is remotely possible, too. It is also possible that you have a Ford 9" rear, although those are mostly in 83 and older Broncos or the 351W H.O. 84-86 models.
For 37" tires, you wil need something like a 4.88:1 ratio. With the six cylinder, you will still be struggling for power or acceleration. That's O.K. if you drive slow or travel difficult trails that need low speed with some torque. Not good for mud or towing, IMHO.
