All,

Sadly, my door panel sticker is gone, so I don't have that for reference. However, the tag on the rear dif is intact and shows:

S842J
2 73 88 4K10

I believe this is 2.73 ratio, 8.8" differential. Please correct me if I'm wrong. It also appears to be non-locking, ie a non-limited-slip differential.

What I'm trying to figure out is gear oil type. I believe it's 80w90, but I can't tell if I'm supposed to add anything to it or not. Also, is the capacity 3qts or 3.75qts?

Lastly, when I change this stuff, is Royal Purple worth the money?

Thanks a bunch for all your help

 

Yes that's a 2.73 open diff.. sorry about your luck on both counts. Correct oil is regular 80w90 with no additives, fill until oil level at at the lip of the filler hole.

 

I also send my condolences too..

 

Why is this a bad dif? Should I consider a swap? I do have a 92 flareside that may have something different.

I'm open to options. I plan to use this as an about-town and towing vehicle.

 

The 8.8" rear axle is fine, it's the gear ratio that is killing the torque. IIRC you have a 300 six cylinder? I would seriously consider going to a 3.55 ratio. If you live where there is snow/ice I would definitely find a limited slip. That could be a factory Trac-loc or more expensive aftermarket.

 

Yep, I do have the 300 I6. Is it possible to swap gears without changing out the whole rear end, or do I have to swap it completely?

So what would a 3.55 give me? Better power to the wheels, better economy?

 

Stole this off the mustang forums

On a 2.73 to 1 ratio, the drive shaft will turn 2.73 revolutions for every 1 revolution of the axle. The engine RPM's vs road speed are determined by the transmission gear ratio and rear-end ratio.

Lets say 5th gear on the transmission is a 1 to 1 ratio (1:1)...that means the engine and drive shaft are spinning the same RPM. So for arguments sake at 2000 RPM, a 3.31 ratio would give a road speed of 49 MPH, the 3.55, 46 MPH and the 3.73, 43 mph.

The 3.73 is more responsive because the engine will always be running a higher rpm in any gear, the 2.73 will have the engine running slower and more relaxed...better for fuel economy and lower RPM' for interstate cruising.

As for gear swapping. You can keep the same rear axle and get the gears and even the gear housing swapped so you could go from your 2.73 open diff to 3.55 Limited slip diff.

Someone correct me though if any of this is wrong.

 

You don't have to change the whole rear axle to get 3.55 gearing but it might be cheaper to do it that way. Setting up gears is not trivial, it may take several attempts to get correct lash, and since shop labor isn't cheap time and materials can add up.

 

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I have yet to try to find a rear diff.

Is the 3.55 the gold rear end? Or should I try to find something else? I've done all repairs based on the premise I'll use it primarily as a tow vehicle, though I've mostly used it for hauling lumber in the bed around town.

The truck feels as though something is holding it back, like I'm not getting the speed out of it that I should, based on RPM. But that's just a gut feeling

 

I like those 2.73 gears… better fuel economy. Ok so you lose some towing performance but you're still OK unless you are doing a lot of heavy towing.

I like synthetic gear oils. You won't need any additives but Royal Purple, Pennzoil, Amsoil or any quality 80W90 will work.