I have a 1989 F150 4x4. I am unsure of my axle ratio's and with good reason. The door code says 18, which I know is 3.08 Non-LS. The guy I bought the truck from told me that he replaced the rear axle with a different ratio. He thought they were 3.55's and replaced the rear with a 3.08, which I have reason to believe the axle is a Limited Slip. I looked for tags on both the front and rear axles and came up with nothing. My question is, how would I go about finding out the gear ratios to both axles? I know I could pull the cover on the rear one and count the teeth on the pinion and teeth on the ring gear and do the math. I don't know if its because I am so used to Chevy's but I can't see how or where the diff cover is on the front axle of my Ford. It is an IFS axle, Dana-44 I believe. I don't have any reason to believe that the front axle has been changed, but I don't want to take the chance and burn up my transfer case.

 

If I am reading right, the previous owner replaced the REAR differential gear, only. Correct?

If the code shows it has a 3.08, and the PO replced the rear with a 3.55, then yea....It'll cause a few issues....

As far as the front diff cover....I'd assume it's on the front of the actual differential housing...

-Wes

 

Jack up both rear tires and with the truck in neutral, rotate one tire by hand. If the other rotates the same direction then it’s a limited slip, if not it’s open diff. If it’s an LS, rotate the tire one revolution (both tires still off the ground) and see how many times the drive line rotates. ~3x it’s 3.08, ~3.5x it’s 3.55. If it’s not an LS, you will have to put one tire on the ground.

Repeat this procedure for the front with the hubs locked in, of course.

 

The D44 and D50 TTBs don't have diff covers. To get inside you have to take apart the hubs and spindles, remove the axles, and then drop the whole center chunk/pumpkin.

 

What happened was the PO thought the gears were both 3.55. He replaced the rear axle only with 3.08 for better fuel economy. I checked the door jamb sticker and it says the gear ratios are 3.08 Non-LS. The only proof of any kind, besides his word, is a junk yard tag for a 3.08 axle, which was in an envelope and not on the axle. But if he replaced the rear with a 3.08 then all should be fine, I would assume. I do not trust his word so I want clarification before I start using 4 wheel drive. The only reason why I believe the replaced rear axle is a Limited Slip is because I did a little burny and had two tire marks instead of one. Also I fishtailed around a dry pavement corner after work today, with little effort. I will try the rotations the next nice day I get. To lock in the hubs, I just put it in 4x4 and they will automatically lock in? I have automatic hubs, btw. Also, the front drive-shaft is taken out due to bad u-joint.

 

Without the front driveshaft, pulling the T-case into 4x4 won't do anything. You can turn the pinion and see if both axles turn or not. If it's non-LS, you can hold one axle while turning the pinion. Same idea as turning the tire, just from the other end.