The Stovebolt Site Forums The Shop Area Driveline Type of rear end in a 1956 Chevy 3100

Hi Folks,

A few rear end questions about my trucks gears. I am not sure how to identify what my gears are. Is there a link to help me find out this that anyone knows about?
Who is the Mfr? Is there a model number from the manufacturer? Everything seems to be original it has a 235 i6 with a sm420 4 spd tranny. I have picked up from this site that I should have a 3.90:1 and if I wanted to hit 70 mph I should change the gear ratio to 3.38:1 from Patricks. Any help on the info is appreciated. Thanks

If you have original equipment.

Rear-end info:
http://chevy.oldcarmanualproject.com/chevyresto/56139.htm

Rear-axle ID:
http://chevy.oldcarmanualproject.com/chevyresto/56139.htm

You'll most likely get a lot of opinions on what to change-to for higher speed. i take a "keep it simple" approach. The 3.38 conversion will get you comfortable 70 mph cruising speed. The SM420 transmission has a rarely used "sub-low" that might become useful with the 3.38 rear.

Thanks a lot its helps me tremendously. Does anyone know who is the manufacturer or is it GM? This is just for my info. I do l ike the keep it simple and will stick with the 3.38

the standard 3.90 rear is a GM item - if you don't have a shop manual, it's available here, worth having

Bill

Yes thanks earlier this week I purchased the 55-59 Truck Factory Assembly Manual (way to cool every spot weld shown), 1955 truck shop manual, and The 1956 Supplement Manual at Tuckers parts in LV. I first reference those books and then come here if I can not find out the info.

You guys are great I have had a couple of problems that you all help me fiqure out that these books could not answer for me.

I guess what I was looking for was there a specific name like a dana40 etc. The books only gives a GM part number. The transmission I found out was a muncie sm420. Thats what I was after. At least I know the ratio and what I will change it to to get better speed on the hwy.

Its my first build and I want to know everything about the truck as I am tearing it apart to restore. been working on it off and on for 2 months now and I still love every day is spring fever. Ok thanks everyone I jsut love the input.

Yours should be an Eaton HO33.

Some parts suppliers, like Randy's ring and Pinion, refer to it as a "55T".

It's always good to be able to get more speed out of these old vehicles but you should also think about the brakes during this process. If you plan to cruise the freeways with the Honda crazies you will need to be able to stop quickly. In my opinion the stock brakes are not up to the task.
Fred

Just putting it out there... An OD transmission swap like the t5 will give you comfortable cruising and 5 usable forward gears. The t5 works best with the 3.90 rear end.

What size tires are you running? Changing to a larger diameter tire can have the same effect as changing the gears to 3.38. For example...
27 inch tires with a 3.38 ratio will act a lot like 31 inch tires with a 3.90 ratio. If you have 27 inch tires on your truck and change to a taller 31 inch tire, you will have effectively changed your gearing by about the same amount, without actually having to tear anything apart.

Thank you that's what I was after. The tires stillhave the original type with a tube. I will change them out to a more of a hot rod kragers look but that right now is last after my paint job.

I do agree that the t-5 from what I read with the OD is a great replacement. I will stick to the original 4 speed tranny for now I thinks its cool. It will not be a daily driver just something for a cruise and going to the store every so often.

The brakes will be upgraded later to power braking system and at least discs on the front. I really will not use it much on the freeways i just do not want want the Clampets on their way to Beverly Hills to pass me.

My priorities are first motor, second rear end, third body and paint, fourth tires and rims and lastly upgrades like a/c, brakes, power steering etc.

You guys have been so helpful thanks to all

regarding a 1948 3/4 ton truck with a 4.56 rear end and original tranny, I changed out the original split 15in rims for 16 inch rims. would 17 inch rims fit the truck and what heigth tires would you use on either 16 or 17 inch rims to partially counteract the 4.56 without doing the tranny rear end switches... that's a great idea

I think 16 inch tires may be easier to get than 17 inch tires. That being said I would look at 235/85R16 tires. Those tires are fairly tall and farly skinny at about 31.7 inch diameter. About the same height, but a little skinnier would be a 750/16 tire. I mention these tires because I assume you would use them for all 4 wheels. The skinnier tires work better with the manual steering. It is also possible to go bigger on the back with 35 inch tires available for most rim sizes. Check your clearences first. I don't own nore have I ever owned a 48 3/4 ton. Check with the local and online tire stores to get an idea of what tires are available/afforable.

I drove the 3/4 ton this weekend at 55mph without any problems for about 70 miles, the 250 cu in is getting about 12 miles per gallon.

once again thank you for your time and good advice