Differential Gears 78 Ford

[78FordTruck78]

I would like to change the gears i have currently. Right now i have a 2.75 rear end and i didn't think i was going to pull my boat out of the water last weekend. If i still want to maintain a good 70mph on the highway and yet pull the boat out of the water what should i go to? I have a C6 with 235/75R15 tires.

 

[1985 Ford F-150]

Id say a 3.50 would be good enough.

 

[DNFXDLI]

I'd look in the 3:31 to 3:73 range.

 

[LEB Ben]

It comes down to what kind of rpm you'll want to be turning. Typically, it's said turning 2400-2600rpm at 60mph is the ballpark you want to be in. That said, you have roughly a 29" tire and at 60, if you swapped to 3.55's, you'd be right around 2500rpm. So if it were me, I wouldn't go any numerically higher than a 3.55

 

[78FordTruck78]

Is it easier to by a rear end with these gears from a salvage yard, or just order the gears new and replace them myself. What is the difference in cost?

 

[LEB Ben]

Is it easier to by a rear end with these gears from a salvage yard, or just order the gears new and replace them myself. What is the difference in cost?

There's pro's and cons to both. If you can find the axle you want with the gears you want, you could probably take it away for under $150. Then you have to worry about the condition of the axle...and your time for the swap. If you do the work yourself on the gear swap, you'll be anywhere from $2-400 into the gears alone depending on what gear and brand you go with. Plus whatever specialty tools you may need to buy. Then you have your time into doing that as well.


Personally, I'd buy new, but I don't do gears myself, because I want someone else to blame and buy the replacement parts if something goes wrong.

 

[SuperCab]

What motor do you run?

 

[Mil1ion]

I put a 380 HP 429 in my truck to take care of the bottom end "yank the trailer out"

 

[78FordTruck78]

I have a 351M in it

 

[Mil1ion]

That's your problem right there.

You have to be careful changing rears. axle shafts may not match.

You need to post info from your axle tag and the tag from the donor to make sure the spline count is the same.

28 or 31 spline !

You are best off changing the whole axle housing

 

[78FordTruck78]

I just dropped underneath the truck and there was a small identification tag held on by one of the bolts. It reads ( I Believe)
First Line DM-8J 7KD
Second Line 75 9 378B
Does this tell you anything?

 

[78FordTruck78]

A local salvage yard has a 3.50 rear end for 100$...should i go with it?

 

[1985 Ford F-150]

Id get it if I was you.

 

[DNFXDLI]

3.50 is a nice all round ratio...that's what is in my truck and it provides a nice all round rpm range.

 

[78FordTruck78]

Great i'll go grab it...thanks everyone for the input

 

[LEB Ben]

As stated...go for it. I didn't decode your axle tag, so I dunno what you have. So if the axle you're looking at came from under a 150, it's probably a 9". If it came from a 250 it could be D60/61. If it came from a 350, it could be a D70. All things to keep in mind depending on what truck/axle you have now.

 

[Mil1ion]

That is a WDM-
that comes in both spine counts.

 

[Mil1ion]

8J ?
Check the 8 again.I think it is a C.

I have a CJ suffix which corresponds to parts list 91 verifying a 2.75 (non-locking) 28 spline axle

 

[money_pit]

That's your problem right there.

You have to be careful changing rears. axle shafts may not match.

You need to post info from your axle tag and the tag from the donor to make sure the spline count is the same.

28 or 31 spline !

You are best off changing the whole axle housing

Changing gears ( ring & pinion ) has nothing to do with axle shafts.

It's a 9" so why change the whole axle ? Just remove the axles & center sections bolts & it drops out. Thats just making more work.

 

[tanman722001]

There is a few different 9 inch.Just like was said the spline count is different and also the axle seals and bearing are different.You will save you alot of work by changeing the complete rearend.It will take you less time to change the complete rearend the messing with changing the center chunk and axles or less you know they have the same seals and axle bearings.I have went threw changeing axles and changeing center chunks in the past.Just my 2 cents

Changing gears ( ring & pinion ) has nothing to do with axle shafts.

It's a 9" so why change the whole axle ? Just remove the axles & center sections bolts & it drops out. Thats just making more work.