11 replies to this topic

[CACTUS JACK]

Does anyone know how to identify a limited slip rear and the ratio on a non-running truck and not able to disassemble? The axle is on an '81 FS Bronco and has a sway bar and it looks like the axle tubes are reinforced out as far as the sway bar attaches.

[Seabronc]

If you look at the Certification lable, located on the driver side rear door post, the axle code is the 5th item on the bottom. For example, if it is 18 that means it is a standard 3.08 differential, if it says H8 it is a limited slip axle. If it is followed by a 2, that means the front axle is limited slip also. If no third character the front axle is a regular differential. the 3.55 ratio codes are 19 nd H9.

Good luck,

[Bully Bob]

Jack it up... if both wheels spin the same direction, it's posi, (or some other form of "TAD")
Do you know it hasn't already had a "ratio" change ? I would rotate wheel once & count the number of rotations of driveshaft.

[Seabronc]

Billy Bob is right, if you don't know for sure that it hasn't had a ratio change, jacking it up both sides and rotating the wheel will tell you what the ratio is. However if there is a tag on one of the pumpkin bolts, it most likely was never changed. Not having a tag does not mean it had a ratio change, it could be just a lazy mechanic failing to put it back on after having the cover off.

80 - 86 were shipped with stock gears of 3.08 and 3.55, any thing else would be an in field change or special order. So rotating the wheel once would be just over 3 rotations of the drive shaft for a 3.08 and 3 - 1/2 truns for 3.55. Others would follow .

Good luck,

[Straight6]

Suppose the wheels and tires are a different size than original equipment?

[Justshootme84]

Rim and tire size will not have any affect on the gear ratio in the rear axle, but it does change the final drive ratio of the axles, tranny and t-case gearing combined.

If the metal I.D. tag is present, the gear ratio code will have an "L" after the "3" on the second/bottom row, like "3L55 8 8 ..." for a limited slip option.

I prefer to rotate the tires two complete revolutions and then count the revs of the driveshaft when figuring the ratio. 7 divided by 2 = 3.5

The F0rd 9" in my 84 has a 3.50:1 ratio, while the F0rd 8.8" in my 88 has a 3.55:1

[Straight6]

The reason I am asking about tire size is if you jack-up the rear and rotate the tires and the tires are larger than OE won't that lead to a higher number of revs of the drive shaft per tire rotation.

[Justshootme84]

No. If the gear ratio is 3.50:1, the wheels will turn once for every 3-1/2 turns of the driveshaft, regardless of tire size.

[CACTUS JACK]

Man, that's some good info - thanks everyone!

[arctcknight]

the tag on my rear axle goes as follows
53180
50 9 6E27

can anyone fill me in on what this means if anything?

[Justshootme84]

It's a Ford 9" axle with open gearing, 3.50:1 ratio. The "3" is sometimes obscured by the bolt holding the tag in place. The other numbers are "Bill of Material" numbers only.