The Stovebolt Site Forums The Shop Area Driveline 3.73 rear gear in stock axle or axle swap?

So I have a ‘59 235ci that I plan on putting a T-5 behind. I calculated a 3.73 gear ratio into a RPM equation and ended up finding an cruising rpm of approximately 2300. Now that’s with an overdrive of .68 from what I found looking up T-5 transmissions. I’m sure they have other OD gear ratios, but that’s the one that I found.

What gear ratio came with ‘59 3200?

What gear ratios are possible in ‘59 3200?

What make/model rear end will transplant easily, or with minimal modification, that has 3.73 gear ratio?

I have searched the forum but can’t seem to choose the correct keywords to find what I’m looking for. I know it’s mentioned a lot here, just looking for the right post or suggestions if you have them.

Thank you.

1959 rear axle information is not available online, but, here is 1958 information.

Thanks guys. So if the stock rear gear, at the lowest is 3.90, I should "theoretically" have a decent cruising rpm with overdrive?

This is the equation im using to determine cruising RPM. MPH x Gear Ratio x OD x 336/Tire Diameter So....75MPH x 3.90 x .68 x 336 / 29 = 2304 RPM (3.73 gear was a little better obviously, but im not gonna be that picky) Of cousre, I havent taken into consideration vehicle weight so thats may be variable to take into account.

Bartamos I found an '87 rear diff with 3.73, did you have to move leaf spring saddle to fit on yours?

Here's a nice gear calculator
http://www.onlineconversion.com/bigger_tires.htm

As far as I remember, I used the donor springs and did weld a new perch and shock mount (shown). I also used the donor shackles and hangers and did a shackle flip, but I was doing 4x4 under it. (That's a whole 'nuther post). You just need to take off the old and start mocking up the donor to plan your attack. I personally would not use and old 3.90 rear end. Why are you doing a T5 and all this for a 235 motor? Are you looking mostly for mileage/gas savings? Don't get all wound up on gas mileage. If you are wanting mostly cruise speeds like 65 MPH or higher, a 3.90 is not for you. If you like shifting a bunch with a 235 6 cylinder, that's fine but did you ever consider a 700r4? or other AOD?

I honestly just love the nostalgia of a straight six and a stick shift for this truck. I’m going to set this up to be driver friendly such as disc brakes, power steering etc. Not really worried about gas mileage.

Well, if the rear gear ends up being 3.70 should I be ok? Or are the old school components not made for those speeds? I may consider an automatic in the future.

Nostalgia would an SM420.

And the SM420 is a tried and true, bulletproof transmission. Used on everything from 1/2 ton to 2 1/2 ton trucks up to and including 1967.

The overdrive T5 ratios I've found are .63, .72, .73, .76, .86. So If you have a .63 od at 60 mpr with 28" tires and 3:73 rear your rpm would be 1834. 4th gear rpm would be 2925. Little high wouldn't you say. However for flat land Texas maybe ok. I'd be looking for a .72 od, that would give you 2106 rpm in 5th with little or no downshifting with the short hills in Texas.

With a 235/T-5 in a '59 truck, you will want the 3.73 or higher ratio (numerically). A 3.90 or even a 4.10 would suit that combo well with a .73 overdrive.
The reason is the lack of horsepower of the 235 compared to a V8.
The beauty of the T-5 behind a 235 is the extra and closer together shifts of the 5 speed, and the overdrive to keep it from screaming on the highway.
Every time you shift, you will be within the optimal power band of the 235. No screaming before a shift, and no lugging after the shift. 70 MPH no problem.
If GM had come out with a T-5 in 1947, there would be no such thing as a car or truck made by any other manufacturer.

That's right, with either o.d., the .63, .72 or .73 the 4th gear rpm is the same ( 2925) given the parameters I laid out above. The only difference is the O.D; The .63 would be 1843. .72 would be 2106. .73 would be 2135. .76 would be 2223. .86 would be 2515. PIck your poison and where you live.

Thanks for all the feed back guys, I really appreciate it. I live in Conroe, Tx so not too many hills around me.

I'd be cautious of the very fast overdrive ratios, you may find the rpm drop between 4th and OD is a bit much.
If the difference between the gears is to much you end up winding it out further in 4th than desired. Or when down shifting from OD back to 4th to pull a hill you (choose to) loose road speed because RPM in 4th is then higher than desirable.
Depending on the application even 0.73 can feel like a stretch.

Try driving a similarly equipped truck if you can before picking a very fast OD ratio.

In my experience 0.8 OD ratio (or less) is very comfortable, you can run along in direct or OD at the same road speed without extremes of RPM in either gear. Nice for hills because its comfortable to run either gear as needed.
You don't always get your pick of OD ratios if axle gear choices are limited, sometimes you need more OD just to get the final numbers to work, then getting between 4th and OD is a compromise.

I hate even to suggest this. You might be better off with a Saginaw 4 speed with the correct 1st gear and forgetting the T5. Then drive it within the comfortable rpm range you are familiar with. After all it's not a race car.

Being this is my first project I'm sure there wil be a lot of trial and error. I wont really know until I have all the details ironed out. I need to figure out what my rear gear ratio is first to see if its something I may use. The transmission I have is a 3 speed. I appreciate all the perspectives and suggestions. I glad to see that there are many options.

I thought of this option as well, not sure what my cruising RPM would be at highway speeds though. I'm guessing 4th gear is 1:1 just like the the 3-speed. If im going to purchase another trans though, I might as well get one that many have suggested and used in the past.

4th gear is 1:1 as you say with a 3 or 4 speed. At 60 mpr with 27" tall tires and 3:73 your rpm will be 3,033. Bit high anywhere. WIth 29+1/2" tires the rpm would be 2824. If it were mine, and I would not be off road driving pick a rear gear that would get the rpm down between 2200-2500 in high gear. WIth 27" tires that would be 3:08 or 3:23.