The Stovebolt Site Forums The Shop Area Driveline Best way to increast gear ratio on low geared 1 ton

I've got what I think is a 48 1 ton pickup drivetrain(title says 1/2 ton but all wheels are 8 lugs). It has a very low geared 4 speed manual tranny and huge rear end diff. Engine is a 235. What is the best way to increase gear ratio so I can cruise down highway at 70mph and 2500 - 3000 rpms? Wouldn't mink keeping the 8 lugs. Do I have to change both tranny and gearing in the big diff or should I give up the 8 lugs and down size diff? Appreciate folks thoughts.

70 MPH in a stock AD truck is pushing way past the safety sensibility limit. I recommend that you drive somewhere that you can't harm anyone including yourself and accelerate to 45-50 mph and slam on the brakes and see how long it takes to stop and how it handles during an emergency stop. Then think about what would happen with the addition of some loose gravel or slick road conditions. I understand what you are trying to do in terms of being able to go more than 45 mph without redlining the engine. I put a late 60s/early 70s six lug 12 bolt rear end with 3.73 gears out of a 1/2 ton truck in my 50 1/2 ton and it is much easier on the engine and I can keep from being blown off the interstate.

You can go one of three ways. One, change the trans to an overdrive, two, change the rear to a higher gear ratio, three, change both the trans and rear gear. Doing the trans swap will probably give you the best overall, though that might depend on what gear is in the rear as to how much you would actually gain.
A gear swap in the rear can get expensive, or, depending what rearend you have, virtually impossible. If you have the Eaton that the 3/4 and 1 ton trucks originally came with, there's not much in the line of available gearing, the 3/4 ton 4.57, the 1 ton 5.13, and the '68-'72 3/4 ton 4.10 (rare). However, if you have a later rearend, like a Spicer 60 or GM 14 bolt, that opens up far more choices for gearing, as parts are readily available for both.
If you do have the Eaton, and want to swap to a higher gear rearend, the Spicer 60 from an '80s-'90s Dodge or Ford van will work fine. The common gear in those is a 3.55. You will need to relocate the spring mount pads.

Fair enough Carl, it's got to be safe. Brakes aren't stock, someone put a big booster on it so I've had it up to 30 or so, speedo doesn't work and it stops short. Appreciate your experience changing out the diff.

Thanks Bill. Seems I'm going to have to give up the 8 lugs and change out the diff w/a 6 lug higher ratio. Which also means swapping out fronts to 6 lugs also. Is there a general consensus among stovebolters on what tranny w/OD works best w/these 235's (is it application dependant, ie type of driving?) Also want to go with a one pce driveshaft. Is there a easy solution to the new setup?

Where to start?

You can keep 8 lugs if you want. There are several suitable 3/4 and 1 ton rear axles out there with better gear ratios available.
Here's a long and drawn out thread (with bad picture links..) about swapping in a GM 14 bolt rear. I think this is the most reasonable swap for an AD 1 ton truck. https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=599879
Also as mentioned you can, if you hunt long enough, find 4.10 gears for the old Eaton axle.

What OD transmission to use? There are a few to chose from, the T5 is an easy swap but not very stout for a real working 1 ton, better off in a 1/2 ton. Then for a working truck the NV4500 is a good swap with a little more work. Lots of threads about both here if you look a little.

What gears or OD do you need to go the speed you want? Not sure you even asked but you should find out first before doing anything. http://www.onlineconversion.com/bigger_tires.htm
Then the next question is are the brakes up to the task with such a higher speed? After that can you even use that speed on anything but a downhill with the current engine, do you have enough power to move the truck through the wind? All things to consider.

Driveshaft. It is made in two pieces because it needs to be to stay straight. If you want a driveshaft that looks more like a banana replace the working two piece shaft with one long slender shaft... Here's the explanation in these manuals http://www.stovebolt.com/techtips/driveline/setup/index.html

Grigg

On my '54 3600 i swapped the rear axle for a Dana 60 out of a 1971 Chevy C-20.
The Dana 60 was only an inch or so wider than the stock Eaton and the only other modification needed was to move the spring perches over some.
The swap let me keep the 8 lugs, a 3/4 ton axle, modernize my rear brakes and get a better rear gear ration (4.10 now).

From what ive read the GM 14 bolt is pretty much just as easy a swap, I just had the Dana available at the time.

Chooglin

I'm assumming your truck has 5.13 gears. Even moving up to a 4.57 gearset, that are plentiful in the old 'bolts will drop your RPMs about 350. Which isn't bad. Going with a larger tire, will help even more. Swapping out to a 4.11 will give you 500-600 more RPM.

Converting to a 6 lug setup in a 3800 truck would be more work than it's worth. The frontend is completely different than the 1/2 trucks. Not to mention, the rearend is not exactly a bolt in either.

I would just upgrade to a 4.57 or even 4.11 gearset.

I have a 4.10 rear and run 235/85R16 tires; 70MPH for me is 3,100RPM. The 4.10 gears for the Eaton seem to be real easy to find, and they are very easily changed out since they are a third-member type axle.

Your transmission should be the SM420; they are still 1:1 in fourth gear, they just have a very deep first gear (therefor the name).

I'd personally just put a 4.10 third member in it and go. Changing rear ends, one piece driveshaft, trans swap, etc, is just making things hard on yourself IMO. (Unless you are planning to drive 70MPH all the way to work every day or something) You don't even (theoretically speaking) need to jack the truck up to swap third members.

Rear end swap isn't that hard to do takes some time at the junk yard with a tape measure.
I'm using a 14 bolt GM 3.73 semi-floater out of a 1 ton van.
you have to weld spring perches on in the right place.
The u joint is even the same.
The T5 out of a S10 will give you over drive.
4.56 and T5 gives a reasonable ratio and fairly easy to come by.
The plus side of a rear end swap is you get bigger modern brakes.

Allen,
I didn't mean to come of all preachy about safety, especially since I don't practice what I preach. I drive mine like I stole it. I just don't want there to be any more idiots like me out there increasing the chances of me dying a fiery death. I still stand by doing an emergency stop test in one of these old trucks even if it is bone stock. It will make you give the guy in front of you plenty of separation.

Bigger rear brakes can be very dangerous unless you beef up the front. It is scary when the rear passes up the front on a panic stop.

x2 on the brake issue. If they are anything like the later trucks the rear brakes are already bigger than the front. Also, if you just find another HO72 (like you already have) you should be able to swap everything you want over from it IIRC. I don't see any reason to swap rear ends unless there is something wrong with yours or you have a free one with the gears you want.