The Stovebolt Site Forums The Shop Area Engine Shop 3rd member pumpkin 41-46 3/4 ton higher ratio?

HI All,
I have two 1946 Chevrolet trucks with the low geared original rear ends. I have heard in the past that a later model Chevrolet truck 3rd member may bolt right in to the stock housing. Has anyone done this retrofit? If yes what year(s) pumpkin (third member)fits?
Thanks for any help.
Brian

haven't done it but I believe it is 67-72 3/4 ton chevy trucks with an automatic transmission. The better ratio is 4.11. Stock ratio for a 3/4 ton is 4.57. Some believe there is a 3.90 out there that will work, others are skepticle.

Bigbadswingdaddy: I have a 3.90 in mine, but a '41 is different from a '46

I'm not sure that 46 the 3/4 tons are the same as the 1/2 ton but I used a 55 pumpkin in my 46 1/2 ton. I think any year truck rear from 55-62 will work. Most of them will be a 3.90.

As I said the 3/4 tons may be different, but it's easy to check.


Good Luck
George

HI
Thanks for the replies. Actually one of the trucks is a 1942 and the other is a 3/4ton special panel. Both are the open type rear end. I will try and locate one to check the bolt pattern...still interested in hearing form anyone who has done this change
Thanks
Brian

the answer to your question will be different depending on lug nuts. If you have 6 lugs it may interchange with 1/2 tons from 55-62. 8 lugs will interchange with 3/4 tons.

Repguy:

Go to the Tech Tip section scroll down to the Transmission and Driveline section. Look through the second tip

Their are 3.90's and 4.10's out there but there rare and hard to find. I have a 1946 chevy 3/4 ton that I am going to mount a 14 bolt axle out of a 1981 chevy 3/4 ton. The 14 bolt has a 3.73 gear ratio. You do know you are going to have to change you speed-o-gears to match the ratio in the rearend. But I do not know where to look for those.

If the truck is running now, you can check the accuracy of the speedo/ odometer with a gps. I would personally pay more attention to the distance traveled vs speed as a 65 year old speedometer probably won't give the right speed anyway. If the odometer is fairly accurate you can compute a ratio for a odometer corrector with the old axle and the new axle and tires. A speedometer corrector usually attaches between the speedometer cable and the transmission.

Well my only problem is find new speed gears for the 4 spped trans in my 46 chevy

Thanks to all. I looked at Kip's41 referral to the tech tips and I know this will solve my 3/4 ton 1942 situation.....sooooo glad to get this info. The 1942 has the 6 lug axles and the 11" brakes. The 1946 3/4 ton special has 8 lug axles and the 14" brakes....so I am hoping the 55-62 3/4 ton pumpkin will fit that unit....now the job of finding those units in the California area.....anyone have one or both? If so what ratios?
Thanks
Brian

55-62 3/4 ton will not improve your gearing.

Brian,
Per 464me: ditto - not sure that 46 the 3/4 tons are the same as the 1/2 ton but I used a 55 pumpkin in my 46 1/2 ton. I too understand any year truck 1/2 10 bolt punkin from 55-62 will work. Most of them will be a 3.90.". Work meaning your axles will match the spider gears just a punkin swap.
I too agree with Big - gearing between a 3:90 and 4:10 is not much difference. That was why I opted for a t-5 with OD. The gear ratio and tire height did not "give enough" for a 70mph, 235, @ optimum rpm. If interested my 46 swap is documented in picts on the fliker link below.
Cavebull

Actually the 46 1/2 and 3/4 ton rears are not interchangeable as far as center sections. My above point is that if the 46 3/4 ton rear was swapped with a 55-62 3/4 ton rear the gears would remain 4.57:1. The Eaton 3/4 ton rear ends will interchange from 46-72. Most of the 3/4 ton trucks came standard with 4.57 gears. It is dificult to find the 4.11 ratio for these rear ends.

Hi Cavebull & Bigbadswingdaddy
Thanks for the input. Cavebull....your photo's are quite impressive. My 1942 3/4 ton has the original 4.57 rear end. I installed a gear vendors overdrive and it gives me 65 MPH at 2700 RPM. I would still like to get to about 1900 or 2000 rpm at 65 mph.....so will try to find a 3/4 ton Eaton pumpkin with the 4.11 gears. Very nice to know the 46-72 will interchange.
Brian

42 3/4 ton is 6 lug correct? You sould be able to swap that one out with the 55-62 1/2 ton center section. That rear will have c clipped axles. 3.90 gears should put you where you want to be, but there is the option of 3.38 available.

Sorry for the confusion. 46 rears are the full floating type. 42 rears are more or less a modified 1/2 ton that has an open driveshaft instead of torquetube. Unless your truck has already been modified by a previous owner.

Hey big,
That IS good to know, for me as well. I have options of 4:11 and 3:38 for my girl if I want to tweak her combo. Thats a way away - but then, that is why we have the archive!
Cavebull