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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old September 1st, 2015, 02:34 PM
gmculater gmculater is offline
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Default 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.
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Old September 1st, 2015, 11:07 PM
Texas63GMC Texas63GMC is offline
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Default 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
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Old September 2nd, 2015, 05:10 PM
jagarra jagarra is offline
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Default Re: Rear end gears

The model 60 Dana Spicer came in 2 ratios, standard was 4.56, optional was 4.10.
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Old September 14th, 2015, 11:04 PM
gmculater gmculater is offline
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Default 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.
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Old September 14th, 2015, 11:28 PM
jagarra jagarra is offline
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Default 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?

Last edited by jagarra; September 14th, 2015 at 11:37 PM.
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Old September 14th, 2015, 11:48 PM
gmculater gmculater is offline
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Default Re: Rear end gears

Definitely new wheels and most likely taller gears also. Changing front end to c10 also.
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Old September 15th, 2015, 12:03 AM
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David R Leifheit David R Leifheit is offline
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Default 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.
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Old September 15th, 2015, 12:56 AM
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Default 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?
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Old April 24th, 2017, 06:01 PM
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Default 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
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Old April 26th, 2017, 02:04 AM
Culver Adams Culver Adams is online now
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Default Re: Rear end gears

Quote:
Originally Posted by gmculater View Post
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.
---
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
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