Rear end gears
 

I have a 1961 gmc 3/4 ton V6 4 speed. I'm wondering if anyone knows what the stock gearing is in the rear end. I would like to change gearing for more top speed. I have a four speed right now and top speed is around 50. Any help or info is greatly appreciated.

Re: Rear end gears
 

you can get a general idea this way: jack up the rear end. Make a line on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the truck, so you can see when the tire has rotated exactly one turn.

Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation. (Transmission needs to be in neutral or out of gear.)

four turns on the drive shaft to one of wheels, is something like a 4.10 ratio
my '63 3/4 ton has a 4.56

Re: Rear end gears
 

The model 60 Dana Spicer came in 2 ratios, standard was 4.56, optional was 4.10.

Re: Rear end gears
 

Hey thanks. I climbed under and discovered a Dana 60 with 4:56 gears. Now I'm thinking about changing out the 8 lug that it has to either 5 or 6 lug. My new question is can I just change axles and if yes what axle, or no I need a whole new rear end.

Re: Rear end gears
 

Changing the axles way not work. Looking at the manual the splines are the same (30), but the kicker is the hub area where it goes into the housing, on the 3/4 ton is 2.117" and the hub diameter on the Spicer 44 is 1.747. Not sure which point that means the axle bearing ID or what. I'm sure others have taken these rear ends apart and will know for sure. I know Brothers sells kits to change the axles to a different bolt pattern, but they run about $500.00 or so depending on model.

Are you trying to get a taller gear or customs wheels?

Re: Rear end gears
 

Definitely new wheels and most likely taller gears also. Changing front end to c10 also.

Re: Rear end gears
 

The only good way to change the rear is the entire assembly. Finding a 1/2ton with 6 lugs off a 60-62 is your best hope. Anything else will require the spring perches being changed. I -had- many years ago a 3:08 which came out of a heavy car and had the spring perch altered for the trailing arm style of suspension. So it can be done.
You will also need a new driveline as the housing on the 1/2 ton is shorter than on a 3/4 ton so a longer driveline will be necessary. So *if* you get an axle from a wrecking yard, get the driveline also.
You will also want the torsion bar assembly from a 60-62 half ton as well, if you are changing the rating.
It is almost easier to buy a rustbucket truck and put your cab/bed on it. We are working on a '66 1/2 ton that blew the rear end, figured easy swap was to 3/4. been stuck with too long drive lines, only driveline I have that would be for a 3/4 ton with 3-speed was out of a '61... the carrier bearing is in a different place so that driveline wouldn't work either.

On a '66 it is leaf rear and coil front so it is real easy to find replacements for. Coil rear and torsion front not quite so easy to find.

Re: Rear end gears
 

Thanks so much for advice. Learning this as I go. So there is no way to fit 1/2 ton axle into my Dana 60 right?

Re: Rear end gears
 

You should be able to custom order axles through mosur in the size and bolt pattern you want. But I would keep it factory and find the wheels you like in the 3/4 ton bolt pattern

Re: Rear end gears
 

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Hello GMCulater,

Here is what I have in my 1961 GMC 3/4 ton 4x4:
Engine: 478 V6, stock (except Edelbrock 2 barrel carburetor)
Transmission: SM420 4 speed, stock
Transfer case: 2 speed Timken Rockwell T221, stock
Front axle: Dana 44, stock (except gears)
Rear axle: Dana 60, stock (except gears)
Axle gear ratios: 3.54 (not stock)
Tires: LT235/85/R16 (about 31.5 inches outside diameter)
At 1000 rpm in 4th gear, it does 30 mph
At 2000 rpm in 4th gear, it does 60 mph
The truck is happy to cruise at 2000 to 2500 rpm for hours on end, though I can use a break more often.

Regards and Hope this helps,

Culver

Re: Rear end gears
 

The easiest thing to do is swapping in 1/2T front and rear axles from a donor 4x4 and selling your axles. Finding one of those for a reasonable price is another issue. Finding one of those with a ratio you want in another issue. If you want a highway driver, you don't buy a 61 GMC V6 4 speed 4X4 and turn it into one. Your truck is worth money the way it is. It's made, and it's ratio is for, four wheeling and power. A work truck, ranch truck, hauler, fun off roading, hunting, fishing with occasional freeway driving. A true original 4X4 GMC, BBV6, two year only hood truck is rare and valuable. Overdrive is another option... or go slower.

I don't know what all the talk about torsion bars/coil springs is all about. It's a 4X4. No torsion bars, no trailing arms, no coil springs.

But if you must: You should be able to use any year 60-66 GMC or Chevy 4X4 and 2x4 (rear) for donor axles with slight modifications. Maybe even more years than that if you want to horse around with it. 60-72 or 73 to infinity and beyond.
Obviously you have to match the ratios front and back. You should be able to use either 1/2T or 3/4T leafs.
Special custom axle shafts or new ring and pinion is an expensive way to go.......real expensive.
I don't think the frame differences/axle widths of 60-62 vs 63-66 will be a big bother in this case. A 2WD GMC donor will have leafs in rear and a Chevy will have coil (most of the time).
If you really don't care about originality and can do mechanical work, the trick thing to do is 70 and up open knuckle/disc brake/205 Xfer case, Chevy/Dodge/GMC. Search internet for more info. Better, stronger, more parts, disc brakes and possibly power steering. I put a 1980 front axle on a 1956 GMC truck. Google 1956 GMC carryall. It will be about the third or fourth picture with saguaro cactus background. Color: 65 Corvette Nassau Blue.