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Username Post: '56 chevy 3600 rear diff options        (Topic#366991)
Posts: 73

Loc: Bardstown, KY
Reg: 06-08-18
11-19-21 05:52 AM - Post#2831101    

I've decided to change my transmission in my '56 chevy 3/4 ton truck to a T5. My rear diff is 4:56 I believe, which right now 55 mph sounds wide open!! Even after the T5 swap, at 70 mph, I believe I may still be looking at between 2800 to 3000 rpm's. Are there other rear end differentials for this rear housing available to get my rpm's down to about 2200 to 2500 rpm's when in the 5th gear overdrive? This is a 12 bolt rear end. I've searched, but haven't had any luck yet...


Valued Contributor
Posts: 4546
Age: 40
Loc: Lewisburg, Pennsylvania
Reg: 12-12-99
11-22-21 01:20 PM - Post#2831280    
    In response to Mike7231

You might have a difficult time finding a ring and pinion set for your current differential. You're probably best to find a later model rear axle that will go under the truck instead. The benefit of the later model rear is that there will be more gear choices and better aftermarket support.

Some people have shrinks. Some people have their garage.

Posts: 66

Loc: NorCal
Reg: 10-24-18
11-28-21 06:34 PM - Post#2831534    
    In response to Mike7231

You're in luck Mike, here's your answer, JoeM is a standup guy to deal with, tell him AlanD recommended him. The originals are VERY difficult to find, akin to hen's teeth.

I'll send the link which is on Ebay to you in a PM, some forums are constipated about Ebay links and I am not sure about ChevyTalk, just gonna be safe. eBay item number: 275037693980

I'm in the middle of converting a '46, so went out and found a '55 rear axle and drivetrain. You should already have an open driveshaft on your '56, should be the exact same axle I'm rebuilding.

I'm using a '95 T5, but it would be easier with a mechanical drive '83-'88 S10 T5. The '93-'95 S10 T5 has a Ford bolt pattern as it's a World Class. GM shipped a Ford bolt pattern those years in the S10 pickups. You need the S10 so that the shifter is far enough forward to get in front of a bench seat, which you probably know.

A lot has to do with the gearing of the T5. I had a closed torque tube drivetrain, so both the driveshaft and pumpkin were useless.

You really need to do the math on each gear in the T5, so to have each tooth count for all the countershaft gears as well as 1-4 (4th is always engaged).

Jay describes the gears pretty well, he's a very knowledgeable guy but hangs out mostly on Stovebolt. I don't use Stovebolt, but exchange email with him.

All that said, the main problem is the inline 6 engines and those old ring/pinion gears in the axles kills them. On mine at 45-50mph you start worrying about the engine/trans blowing up, at least I do. As GM moved into the 50s they were able to compensate with the V8 engines, as they seemed to be handle the RPMs better than the Inline 6. I have no idea what engine is in my '56, but don't think they used the 265 (first V8) until about '58.

Hopefully some of that makes sense to you. There are many gear ratios and T5s from cars to trucks, etc...people even modify them for Jaguars.

EDIT: Sorry, forgot to add Jay's video. As you watch this keep in mind that 4th gear is 1:1 ratio, and most all transmissions will give you 1:1 in the highest gear, even on the old 3 speeds. So you really need overdrive to take you under 1:1 and the real question becomes how much your engine will be able to carry on how low of a ratio that overdrive can be. In my case I'm using .86 on the overdrive. But fortunately these 3.38:1 gears are even better than the more common 3.55:1 for the '41-'53 trucks with the torque tube. In short on mine overdrive will be able to take it 14% lower on RPM (i.e., RPM * .86). Between the ring/pinion ratio and overdrive is where it mostly effects the highway speed on the top end. Also, keep in mind that I'm using an Inline 6, the V8s and the newer automatic transmissions were capable of higher RPMs. That transition was happening around '58 as I recall. I'm describing the common problem with the standard gearing in the pre-'58 era trucks being unsuitable for driving on modern highways. (that outta rile some feathers on the Internet... )


"Keroppi" - 1946 Chevy 1/2 Ton Pickup

Edited by Keroppi on 11-29-21 01:13 AM. Reason for edit: Forgot to put the video in this post

Posts: 73

Loc: Bardstown, KY
Reg: 06-08-18
12-04-21 12:23 PM - Post#2831897    
    In response to Keroppi

Thanks for the response, and sorry for the delay getting back to you. I will email and check on the gears. The problem I have is, this is a 3/4 ton truck. The 1/2 ton truck has several gear options. I may have to change the entire rear end to a 1/2 ton, to do what I want. I'm just getting started on my restoration, and still changing is a minimal problem on this truck, really great shape from that standpoint. My gauges and speedo are junk, so I'm going to go Dakota Digital, so doesn't matter much on mechanical or electronic speedo in the transmission. All the wiring will be upgraded with a new harness, with more options available. I just sent the block and crank to machine shop to see how much wear, hopefully won't have to bore more than .030, and the crank looked really good, hopefully won't have to bother the journals. I'm going to start the the restore in full swing after the 1st of the year. Still debating on who will repair the minimal rust. It wont take much, but I'm not a cut and weld person, so someone else will tackle that...

Posts: 10

Reg: 12-04-21
12-04-21 08:21 PM - Post#2831924    
    In response to Mike7231

I built a 10 bolt from a '79 Camaro, and installed it under my '59 Apache. It has worked great. Parts widely available for it. LS1 disc brake swap was cheap, added a locker, 4.11:1 R&P and chromoly axle shafts to mine. Torched off old brackets, welded new spring pads on, and it has been great! I only paid $100 for the housing at a pick 'n pull yard. I may be pulling it soon, to swap to a Jaguar independent rear suspension that I picked up, but still undecided. There are plenty of guys that have swapped to the Jaguar IRS and are happy. Other than that, I'd look for a Ford Explorer 8.8" housing! They come geared with 3.90 or 4.10 gears (I think) with rear disc brakes, and are a pretty good width for these trucks! There are various websites that list rear differential widths from various vehicles, that would be a good match to your truck. I would not worry about keeping a rear differential original, personally. Unless you have some very special, rare optioned concours truck, it just doesn't effect actual value of the trucks.


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