The Stovebolt Site Forums The Shop Area Driveline Is it worth changing out diff gears?

'59 235 with the factory 4-speed and a limited slip diff that, if I counted the revolutions correctly, is a 3.90. The brand new tires that came with the truck are 235/75/15's.

How much benefit will I get from changing the diif to a 3.58? That's all the guy at Arizona Differential says he can do with that diff and he charges $800 for the work. My goal is increased cruising speed (70 mph?) so we can use it for our annual cross-country vacation as well as occasional freeway driving locally.

I don't understand diffs, suspect they're magic of some dark sort, so can't tell if the cost/benefit ratio makes sense.
Thanks.

If you have the factory limited slip differential, the guy at the shop you talked to may have mislead you. The trucks with the limited slip option in '59 used the GMC Spicer 45 axle with a 3.92:1 ratio (the regular open GMC axle could also be had with a 3:07 and I believe a 4.11 gear) with no option. Parts, other than seals, are virtually non-existant for this rear, there are no gear sets being made as well.
A better way to go would be to consider an overdrive transmission, possibly a T5 5 speed or MY6 4 speed if you want to keep the floor shift. The overdrive gear, coupled with the larger tires, should give you the performance you're after.

A newer 5 speed transmission would probably get you closer to your goals. A T-5 has an overdrive in 5th gear.

John

He says he'll swap my limited slip for a standard diff with a small credit. I've read that wheel cylinders for the LS backing plates are hard to come by, so that's a swap (with the gear change) that seems worthwhile. A change to a T-5 is not something I'd considered, but will!

I know the T5 has been the mainstay swap. I just recently learned of the Aisin Transmissions. Jeep used an AX15 in the 2wd Cherokee, Toyota used a R154, and GM used an AR5/MA5 in the Colorado and Canyons. The NV 3550 is also based off of the Aisin.

Basically, the only diffence is in the input shaft and bellhousings. The Jeep version used the same 10 spline GM style that they've been using for years. The AR5 uses a 26 spline that GM uses in the newer Camaros, and Toyota does their own thing.

With an adapter, you can add them to just about anything. They seem to be a little more robust than the T5. Parts are little scarce, but they're out there.

http://www.novak-adapt.com/catalog/adapters/engine_to_transmission/ax15/kit_gmax15.htm
http://www.advanceadapters.com/prod...p-wrangler-ax15-adapter-bellhousing-kit/

Just some more food for thought.

I'd keep the stock ring and pinion and do the overdrive. Low end grunt with a nice highway cruiser.

A small credit? In other words he wants you to give him the limited slip and he'll make it look like he knocked a little off the labor.

As far as wheel cylinders go, cylinders and shoes from the standard corporate rear end can be used in the Spicer. I've been running them on my '56 with a 3.07 Spicer for years. The drums will not interchange however.

I agree with those who recommend an OD. My preference is the SM319.

+1 on the SM319.

My 57 with 3.90 gears and this OD trans is a whole new truck. You'll be amazed at how much quieter and fun it is to drive. And can probably locate and rebuild one a lot cheaper than what you're thinking on paying for the diff swap.


Jim

I'm also a big fan of early OD's. In addition, later Saganaw 3sp OD's and T5's are a good way to go. However, you need to do a cost comparison between doing the rear end and switching to an OD.
Any transmission change will require a driveshaft modification for your particular truck.
3 Speed OD's will require either the column shift linkage or a modified floor shift set up. You will also need the electrical components.
T5's have issues as well such as location of the shift tower, clutch replacement, and some of the later units are designed for electric speedometers.
All of the above can be overcome but they are things to consider.
Fred

70 mph is awful optimistic. I put big tires and a 3.5? rear end in my '54 and 55mph is really pushing things. Problem is the valves float.

If you are going down a steep hill and take you foot off the gas you will notice very little compression braking. Almost freewheeling. As you bleed off speed you will notice when the compression comes back. Valve float under load is a quick way to burn valves.

My owners manual (1954) states plainly:
"Sustained speeds greater then 45mph are not recommended".

Is that quote from the 1954 Chevrolet Truck Operator's Manual or the Car Owner's Manual? I see a warning in the car manual about not towing over 45mph?

I have a 3.55 rear in a 54 truck with big tires (diameter 31.5) and it will cruise all day at 65 mph - do you expect "valve float" at 2500 RPM?

I have cruised at 70 for four hours - less than 2700 RPM. I might not recognize "valve float" but my professional mechanic friend and racing friends might have (if it was occurring).

My mechanic-friend was also impressed by the smoothness and power when we cruised for a short while at much higher speeds.

Even then, that warning is for towing a passenger car equipped with a Powerglide.
I'm running oversize tires as well (30 inch) and have run 70+ with no "valve float".

How many illegal mind-altering substances are you on?!? You aren't even close to valve float. I don't have a 235, but I couldn't imagine valve float happening at less than something like 4,000RPM (I'd guess higher). You have to be running less than 2,500RPM! There are lots of reports of guys with full-pressure 235's running well beyond 3,000RPM on the highway with zero trouble. What about a grain truck? Forget about driving speed; how are you going to pull the wide gear changes of an SM420 in a heavy truck if your valves float and your engine quits making power at 2,500RPM?

I have 4.10's and a 32" tire on my current truck, and my slightly later 250 six would purr like a kitten running 80MPH if I wanted to pay for the fuel! The later sixes are a little better/stronger, but not by that much. I normally drive 55-65MPH; I could exceed 55MPH in third gear before the valves float!

BTW, valve float under load isn't "a quick way to burn valves", it is/would be a quick way to drop one into whatever cylinder has the lamest valve spring and blow the engine up.

After following this thread and and doing more research the plan is to install a T-5 from a mid-80's S-10. Mechanical speedo, minor modifications to the drive shaft, and all the advantages of an overdrive. Slightly more expensive than the diff mod but it seems like a lot more bang for the buck.

I'll probably be back for help with the install when the time comes.
Thanks for the counsel!