VCCA Home Forums TECHNICAL - GENERAL Trucks and Vans High speed rear end

My friend has a 1950 GMC 3/4 ton with a 4 speed transmission that his father bought new. It is very slow on the road and he wants to be able to go places with it. Does anyone make high speed gears for it, or a rear end conversion?

Thanks

Hi Ross,
I'm just down the street from ya, take a look at your PM's
Denny Graham
Sandwich, IL

Gears from a 3/4 or 1 Ton truck up through 1972 are supposedly a direct swap. (entire drop out unit) I have not done this myself.

Mike

Check with Patrick Dykes for 3.55:1 ring/pinion gear sets

1/2 ton only Mac, I just made the switch to a 4.10 centersecton this summer, cruise at 55 now with out worrying about tearing up the engine.
Denny Graham
Sandwich, IL

Patrick's also has a 4 speed floor shift for the truck too, I think the final drive was like .83 to 1 or close to it.

Denny, what did you use a center section out of a 70s
3/4 ton with a manual four speed? What was the ratio of your original rearend. I thought that most of the old advance models had a 4.11, or was that just 1/2 ton trucks?

The 1939 3/4 ton had 4.11
The 1941 3/4 ton had 4.56
The 1949 3/4 ton had 4.57 and 1 ton had 5.14.

just some exmples

When the later (like 1970) trucks came along with 8 Cyl engines and automatic trans. they went to a 4.10.
The ratios are not as bad as they sound because the truck tires were of a larger diameter.

As far as noise goes at speed the truck cabs had little sound proofing, no interior cloth to absorb noise (just lay a piece of carpet on the floor and see what an improvement it makes) and a larger fan that made much more noise.

A 1/2 ton truck at 60 MPH sounds like its in 2nd gear compared to a car at 60 MPH with the same engine speed/gear ratio.

Thank you Gene, I agree with you completely on the sound inside the AD cab. I have the original firewall pad, which is still intact, all the firewall penetrations have new rubber or felt and a new carpet pad and new “rubber mat” and at 55-60 mph it is quite noisy. The temporary tachometer I’ve installed tells me that I'm only at around 2400 to 2600 rpm at these speeds but it is noisy. It's been 45 years since I've been in a car of the same vintage so I couldn't make a comparison. I just know that I often find myself reaching to shift into 5th gear.
Please correct me if I’m wrong in any of this, you are the expert after all and much of this just mirrors what you posted earlier. The original question was about a GMC and I can only comment on what I’ve found with the AD Chevy’s.
Mac, this is my understanding of what was available during the AD period. According to the replacement parts available in the parts books and from what I have seen.
In the cars from 1940 to ’52 the regular production option was a 4.11 an optional 3.73 was available. With the PG introduction in 1950 they came with a 3.54 up to ”˜54. The 1953-’54 standard shift changed to a 3.70.
In the trucks the 1/2-ton was a 4.11 from 1940 up to ”˜53, the 1954 came with a 3.90. Everyone knows what happened in ”˜55.
In the 3/4 ton from 1946 all the way up to the late 60’s the standard ratio was a 4.57 for the two-wheel drive trucks equipped with the Eaton HO52 rear end.
The standard gear in the 1-ton was a 5.14.
The center section that I got my 4.10 out of was a 1970 3/4-ton V8 automatic. It is my understanding that this gear ratio was available from 1968 to 1972. Except for the gear ratio and a few minor changes inside the differential carrier is identical to my original 1950, 4.56 and use the same axles and drive shaft. Everything internally will interchange, that is the case, the spiders, the ring gear, the side bearings, and the thrust pad screw. The pinion and the pinion rear bearing support are different so the pinion is not interchangeable. Of course the ring and pinion must be installed as a set so that means that although the ring gear will fit the old carriers the pinion will not, making it impossible to just change the R&P.

Denny Graham
Sandwich, IL

In 1950-51 I would drive many of these trucks when then were fresh from the factory. Would drive them to our storage building or do dealer trades, take the larger trucks to have bodies installed, etc. The front floor mat would not be installed and you would be surpried as to all the noises you could hear, including from the drive train. The panel trucks had a completely different sound. The hollow big empty body amplified the noise and sounded terrible.

It was mentioned above about the tire size. I've seen many of these old truck that have 15" wheels on them and the tires that are put on them are really small and some have had 16.5 wheels put on them and these end up with tires that have a small diameter. You may want to look at putting a 16" rim on it with a tall tire. Thats a easy way to change the gear ratio without actually changing the gears. I've also found that if you use fresh tires on these old car and trucks it makes a HUGE difference, and a bigger difference if you use a modern steel belted radial type tire.

Good Luck
Don

You might want to look at the chart on top of web page <http://home.znet.com/c1937/RPM.htm> to choose tire size and gear ratio.

Thanks I will tell my friend.