The Stovebolt Site Forums The Shop Area Driveline Want to go with a 4 speed

I have a 1955.1 Chevy 3100 with the inline 6, 3 spd on the floor manual transmission. I do not think the transmission is original, but believe the rest of the drivetrain to be original. I would like to have an overdrive option or just to have her drive on the highway better. I am currently thinking a 4sp, but open to a 5 spd or better. My question is, is there a 4 spd or better transmission that I can bolt in, with minimal part swapping? I really would prefer not to have to purchase a new driveshaft, etc.

Thanks,
BH

The standard “bolt in 4 speed” is 1st gear crawler 2-3-4 are pretty much the same as the 3 speed ............no overdrive 4th gear.

Modern Tremec 5 speed would get overdrive........but requires modifications etc.. google Tremec 5 speed for hours of reading.

One of many prior posts on the topic:

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/453918/Re:_what_transmission_will_fit.html

BigHog, This is the link to my S10 5 speed conversion. This thread addresses the good T5 I installed since the original $100 junker was just too noisy for my taste.

My recommendation is if you want a driver that's a pleasure to drive, engine rpm wise, do a T5 conversion. It's fairly straight forward especially with all the help available. You do need a place to work, preferably in a shop or carport and basic hand tools. A transmission jack or at least a floor jack are very helpful. This site has a how to as well in the Tech Tips section on the home page, top left.

My conversion has about 10k on it with a couple 1,000 mile plus trips. No way I'll go back to a stock ratio drive train.

Good Luck, and check out member lugnutz site. It's linked in his tagline on all his posts.

RonR

Couple things to also consider is what size gears are you running in the back. Also with your desire to drive faster, will you have the brakes to stop at faster speeds.

A 55-1 series truck would have an open driveline. The original transmission would be an SM318 Muncie with the 4 bolt side cover. The floor shift conversion would be aftermarket. The stock rear end would likely be a 3:90 ratio. An overdrive transmission was available from 55-65 which is a direct bolt in and will give better highway speeds. There are numerous options available including changing the rear end gears.
Fred

Hi BH and welcome to the place,

As the two guys before me said, the T5 will be a really good option but depending on how you plan to use the truck, you may have to swap out your driveshaft and rear end. We don't know what ratio of rear end you have at this point, and we don't know what transmission gearing you have. So, let's look at one option. If you can find a 3.07 or 3.08 rear end and a T5 with these gear ratios:
1st gear---4.03
2nd gear---2.37
3rd gear---1.49
4th gear---1.00
5th gear---.86
(There were lots of these out there. Mostly they were used in S10 trucks or Isuzus with either 4 or 6 cylinder engines. 1982-1984 as I recall.)

The rear end with a 3.07 or 3.08 ratio.... common in 12 bolt rear ends and some used in late 60s or early 70s pickups were the right width. Your driveshaft may or may not work...you'll have to see about that. The T5 is roughly 25 inches long from where it bolts to the bell housing to the end of it.

Ok, if you find these two combinations, here's what you'll have as your final gear ratios:
1st gear---4.03 x 3.08 = 12.41
2nd gear---2.37 x 3.08 = 7.30
3rd gear---1.49 x 3.08 = 4.59
4th gear---1.00 x 3.08 = 3.08
5th gear---.86 x 3.08 = 2.65

And these final ratios will go up/down based on the diameter of your wheels and tires.

So if you still want to be able to put 1/2 ton of "stuff" in the bed and drive like a pickup should drive, the above combination will work great for you. It is very close to the ratios the original GM engineers wanted in there, so for the first 3 speeds, you'll feel like nothing has changed except your first gear will be synchronized. There are other ways to skin the cat and I'll mention one of those in a moment. Then in 4th gear, you get your first overdrive gear (in my case it was a 25% reduction from the stock 4.11 rear that was originally in the truck). Then on nice flat or slightly hilly roads, in 5th gear you get your second overdrive gear (a 14% drop) and you'll be cruising (60 or 65 mph) at 2000 to 2200 rpm depending on your tire size, etc. With a 235 I really don't believe you want to cruise at less rpm than that. Torque maxes out around 2000 and that's a good spot for you. You don't want the six to be lugging.

If you have a 3.55 rear end, you will do better with a T5 that has the following gear ratios:
1st gear---3.50 (x3.55 = 12.425)
2nd gear---2.14 (x3.55 = 7.597
3rd gear---1.39 (x3.55 = 4.93)
4th gear---1 (3.55)
5th gear---.73 (x3.55 = 2.59)

You can see the final gear ratios change a bit from the 3.08 scenario but stay in the same ballpark. The 5th gear ratio is a bit lower than I'd like, but a slightly smaller tire diameter would solve that if needed. The bad news is this transmission might be harder to find. As I recall this one was only used in 85 and 86 minivans with V6 engines. I think there may have been more T5s made with the 4.03 first gear scheme, but I could be wrong on that.

Or, you may be able to change your rear end gearing to 3.38 and go with one of the T5s with the 3.76 first gear...as I said different ways to get to the same spot. You just want to be very mindful of the first gear overall ratio...some would call this your SLR or Starting Line Ratio and other people might call it your takeoff ratio. With a pickup and an inline 6 I believe you need that to be at the minimum 11 or above. Otherwise when you start off there won't be anyone home under the hood and your clutch won't last long.

Important note: you may have to settle for a T5 with an electronic speedo output. Those with mechanical output are getting hard to find and costly. If you do, don't worry. There are ways to overcome this. If you look in the electrical bay section of this forum, you'll see where I (and a couple of other people) have been doing work and explaining how we're doing this. And it is possible to have the electronic speedo look pretty much like the original. So, you do have choices. Good luck no matter which direction you go.

Welcome BigHog. A 55.1 is in many ways a good choice: the last and best of the Advance-Design and the first with an open drive line in 3100s. As others have stated, it depends on what you intend to do with your truck. Driving a little faster can be fun, but safety first!

There is a vendor or two on eBay that offer mechanical speedo conversions. Pricey and no idea on the quality but it’s an option. I was lucky enough to find a T5 on kijiji only an hour or so after being posted. I took a risk and the seller held it till a coworker could pick it up. My other option was buying an S10 with an electronic speedo and convert it to mechanical.