The Stovebolt Site Forums The Shop Area Driveline 57 Hydramatic rearend gear recommendation

Hi,

I have a 57 Chevy 1/2 ton that has a Hydramatic. I have never ridden in a TF truck with Hydramatic, can any one comment on how it drives? Im debating on changing the rear gears to 3:38's, does anyone have this setup? Right now I have a 3:90 rear, what would the rpm's be at 65mph with the original rear. Im torn on keeping the rear end original, but also want to be able to drive it at "highway" speeds even now and then.

Thanks in advance for the input!

There are several programs on the 'net to use tire OD, etc. to calculate RPM's at certain speeds.

Ed

The dual range Hydramatic has a very complex balance between main line hydraulic pressure and throttle pressure, so tinkering with the RPM/road speed relationship just might create some unintended consequences with the shifting. Proceed with caution! A taller tire might be a better approach to getting a little more road speed without messing with the rear axle ratio.
Jerry

I went to see a guy who spent almost $30K on a truck and then put it up for sale. A gorgeous, totally rebuilt and original truck. I asked him why he was selling it. He said he and his wife went on a trip to a car/truck show. He won first prize. On the way home his wife said she didn't like riding in the truck. It was too noisy and rode stiff.

The Hydramatic doesn't have a torque converter. GM designed-in a low first gear to get the truck moving. First to second happens quickly. Changing from 3:90 to 3:38 may affect the truck initially moving. I suspect it would be okay unless you were hauling a load.

Is your truck V8 or I6?

Hi,

Thank you for that input. My truck has a 265 v8, we pulled all the running gear and other misc parts from an original Hydramatic truck. My truck originally had a v/8 4 speed with granny. I spoke with a longtime Hydramatic rebuilder and he said that the trucks came with a higher gear ratio due to this tranny. Based on aski f the question before here, see I g the options in the service manual and spinning the tires in the rear it, I determined it was a 3:90. I’m at the point of taking all suspension pieces off and sending the rear to be rebuilt thus curious about weather to change the ratio or leave it.

Thx

My buddy has a 56 GMC Suburban Carrier with the stock 316 Pontiac engine and the original hydramatic transmission.
It also has the 3.90:1 rear end. That ratio is way too low for that truck, it is in 4th gear at about 32 MPH and sounds like it needs one more. First gear on the hydro is 3.82 which is lower than the stock 3 speed first gear at 2.94 and second gear on a SM420 at 3.58. If it was mine, I wouldn't hesitate for a minute to change to a higher ratio. Oldsmobile's, Pontiac's, and Cadillac's of that era used the same transmissions and they didn't use anywhere close to a 3.90 rear end.
Fred

Thanks Fred. When looking up these gears looks like there are 2 on the market. One that tom diff sells and one from Yukon. Does anyone have any experience from either?

Thanks again, this is a big help!

OK- - - -more information is always good! With a 265, you should be able to handle a higher ratio than the usual 235/automatic setup. Do a little research on the axle ratio a mid-1950's Pontiac or Oldsmobile ran, and gear your truck accordingly, especially if you're building a highway cruiser instead of a load-carrier.
Jerry

The 265 V8 is a great little engine. You did not specify but make sure your donor transmission is from a 265 V8 truck with Hydramatic. A 6 cylinder Hydramatic is a different bird.

Please check but I think the transmission gearing for car Hydramatics are different from a 1/2 ton trucks. Also bigger (1-1/2 and 2 ton) trucks had a lower first gear.

It also depends if you want a truck with all original parts (for show etc.) or a truck to drive around in for fun.
I don't know about the 57 but earlier 54/55 Hydramatic trucks have a different floorboard, starter, master cylinder, cross member and carburetor etc.
Personally, I would do the rear axle much later until you had a chance to make the truck in good running condition with the 3.90.
Your "Hydramatic rebuilder" should have the tools (throttle arm setting gauge, band adjusting tool, hydraulic pressure gauge and how-to-manual) to set it up.
You need to experience what a normal running/shifting truck is like (establish a baseline) before you change the rear axle.
Then it's like Jerry said, it will need tinkering.

Lots of great advice, thanks! I bought the the whole drivetrain from an original 57" Hydramatic truck (he is restomodding his truck), which included, engine, transmission, front suspension, rear suspension, rear end, steering box with shifter. He is saving the floor access plate for me. Was the carb different on automatic trucks. Does anyone have a picture of a 57 Hydramatic truck to confirm the one I have is stock or not.

I heard from a rebuilder that the rearend gearing was different, but after testing, the one that was on the truck is a 3:90.

I was stating to take the suspension parts off the rear and noticed its wet in the back and front (pinion) so I thought it would be a good idea to rebuild it. I think your right approach would be to rebuild the original one, see how it drives and decide to change it later if needed, like you said (It will also allow me to save 300-400 on new gears which would be nice).

Thanks all!

We did an original restore on a 57 Cameo and it had a 3.55 behind the Hydramatic. The 57 trucks came with 265's even though the cars went to the 283. It was geared PLENTY deep but would be much more cruise friendly than a 3.90. You can see the "V" with "Hydramatic" on the door emblem.

[img]https://i.postimg.cc/d7tnjt3j/Cameo002.jpg[/img]

Hi @coilover,

Were those stock 3:55 gears behind it, or was it changed to that somewhere down the line? Do you have a build thread with pictures, I would love to see pictures of your restoration. How did the car drive and shift?

thx

If you are going original here are some things to check before you spend the $$$:

HT = Hydramatic Transmission
Engine and HT may be a matched pair (some HT have engine serial number stamped on a HT tag).
Engine may be stamped with code letters indicating HT. (http://chevy.oldcarmanualproject.com/models/engine.htm)
Using another engine may require checking the alignment of the housings and check the flywheel and torus cover run-out.
(https://www.stovebolt.com/ubbthread...ans-front-rear-housings.html#Post1355625)

Make sure you have a good/correct VIN tag (door post) that agrees with your title.
VIN stamped on frame (https://www.trifive.com/forums/showthread.php?t=115637).

Hi thanks @buoymaker,

I looked at the engine code and it was on the front pad and it ended in LA, so it looks like its the matching engine to the trans. I will look on the trans for a serial number. Once I get the cab off the frame, I will double check the vin matches the frame. I like to keep things period correct, it will never be numbers matching since I purchased the whole drivetrain from another truck and swapping it on my truck. My truck is an original V8 4 speed w/granny gear truck though so the swap should not be bad.

Thanks for all their info, I have sure learned a few new things!

In case you didn't know, there is documentation online. The quality is okay but I prefer the books.

57 Operators Manual:
http://chevy.oldcarmanualproject.com/chevyowner/57tomindex.htm

Couldn't find 57 but 55 2nd Shop Manual may help:
http://chevy.oldcarmanualproject.com/shop/1955truck2/index.htm

55-59 Factory Assembly Manual:
https://www.trifive.com/55-59Assy.pdf

29-57 Master Parts List:
http://chevy.oldcarmanualproject.com/1929_57chevyparts/index.htm

There's nothing in the chassis serial number that ID's the engine/trans package other than a prefix "V" for V8, but the "V" and the Series ID will not be on the frame. The frame will have the Year, Plant and Unit Number...the Series can be ID'ed by it's physical characteristics of the frame.

Your engine will have a "code" providing the Plant, Day, Month and the series of the vehicle it's going in and trans it's coupled to.

Mike B

There may have been more aftermarket selections back in the day but the only one that I know of currently for TF trucks is the 3.36 gear set. According to my "1956 Task Force Engineering Facts Book" the only factory ratios available tor a 3100 and 3200 were 3.90:1 and 4.10:1. Possibly 57's had something different but I don't have any reference material for them.
There was a 3.07:1 rear end ratio available For GMC 1/2 ton trucks of that era. However, it is a completely different rear end assembly from the aforementioned Eaton rear end. The housing is different, the gears don't interchange, and even the brake drums are different. Probably not a lot of them left in captivity. I think something similar was available in some 59 Chevy trucks that were ordered with positraction.
Fred

Cozsum,
I just have a few pics of the truck that I had paper photos digitized. There are some on my old computer with the XP system but I'm not smart enough to recover them. The truck was very rough with the dash cut out and the center of a Ford dash screwed in just to have a radio I guess they already had. It was a driver only truck since it had a 383 Chrysler engine and the right side exhaust was where a passengers feet would be. The customer we restored it back to original for entered it in shows and it nearly won it's division every time. It's since been sold several times. As far as I know the 3.55 rear end was original but since the truck couldn't talk I can't swear to it.

Center of a TF dash welded back in where Ford section was:
[img]https://i.postimg.cc/k2sXvVsC/Cameo006.jpg[/img]

383 Chrysler engine

[img]https://i.postimg.cc/hXZhqzdr/Cameo005.jpg[/img]

Trailer load of pieces that came with truck bolted together for test fit:
[img]https://i.postimg.cc/kVtM3j9J/Cameo004.jpg[/img]

Finished bed and rear of truck
[img]https://i.postimg.cc/PpQLptxV/Cameo003.jpg[/img]

Wow very nice work! Mine is a 3:90, I measured it by spinning the drum 10 times and counting the amount of times the yoke turns and multiplying by 2, then moving the decimal point. My also had an AF marking on the face of the diff (9 o'clock position). Im going to have it rebuilt so ill get a better look when its taken apart.