The Stovebolt Site Forums The Shop Area Driveline 3:55 rear

I was considering doing the swap to 3:55 rear in my 54 GMC. My only concern is how would it operate if in the future i change it from 3spd to four or more on the floor?

Give this guy a call. http://www.patricksantiquecars.com/ He is the one who developed the 3.55 gears over 20 years ago and he can answer any questions you have.

Check the ratios in this table. The GMC ratios are the same. I find starting off in second gear with the four speed to be better than starting off in first gear with the three speed (when running with the 3.55 rear, which results in lowering the ratios by about 10%).

http://chevy.oldcarmanualproject.com/chevyresto/54156.htm

He might consider a later model Muncie 4 speed out of a passenger car. Patrick has a kit to adapt these to the torque tube. I agree that there would be no point in installing a SM420 in front of 3.55 gears.

Im fairly new to this and would like to know the difference between SM420 and the Muncie 4 speed. Also why stop at 4? as opposed to an overdrive gear? Is it an impossible swap?

The SM420 was standard in 2 ton trucks and an option in pickups. It has a very low first and reverse gear. It has no purpose other than starting heavy loads such as a trailer. Your 54, if it has the components it was born with, will have a 248 engine with 3.90 rears with a closed drive shaft. I am assuming you have a 1/2 ton 100 series. My favorite way of getting more speed out of these things is using an open drive shaft rear out of a 55-59 which is also 3.90 and then installing a 3 speed with a BW overdrive. This gives you a .70 overdrive. 3.90 is a good match for the OD. Anything faster would make OD only good for going down hill. Cletis has a Suburban with a 248, 3speed OD and 3.90 gears and he put over 10k miles on it last year. However, I think he likes the smell of rubber burning and it may have a 302 in it one of these days. The Muncie 4 speed is a closer ratio 4 speed that was used in muscle cars. However, 4th gear is direct and you don't go any faster than what you have now.

got it. So a good first step would be an open drive shaft? Basically the 4 speed will not really improve cruising speed if this first step is taken? Also with this open drive shaft is it being used with the 3:55 or the stock setup?

didn't GMC have an open drive in 54 like Chevy did?
any 4 speed won't "improve cruising speed", no matter what rear gears you have, both 4 speeds you're talking about [and a standard 3 speed] are 1:1 top gear .... you can get 3.55 gears for the original carrier in your 54 axle which would mean you don't need an OD, but stock it would be a 3.90, which as crenwelge says is best for an overdrive, even if you put in a modern one like a T5 or automatic 4 speed where 4th is OD

Bill

The 1/2 ton drive line in 1954 was enclosed in all GM trucks.

However, the 3/4 ton and larger trucks had open drive lines from the transmission.

ahh, my misteak, I always think of the 55.1 as 54's

The 3.55 conversion must use a '53 or earlier carrier but it will fit in the '54 housing.

i was told that my truck was a 3/4 ton how do i confirm this?

If you have a metal ID tag on the cab between the driver's door hinges, post the code here. Someone will help you figure this out.

How many lug bolts hold the wheels on. If it has 6 you have a 1/2 ton. If it has 8 it is a 3/4 ton. All of the previous posts have assumed you had a 1/2 ton because you referred to a 3.55 gear set. There is no 3.55 gear set for a 3/4 ton. If it is indeed a 3/4 ton, the only practical way to go faster is an overdrive transmission. If you are not sure if its a 1/2 or 3/4 ton I think you may be getting ahead of yourself in your plans.

just read your post. i have a ring and pinion gear ordered from patricik's in arizona just to "improve cruising speed" by lowering the gear ratio. do i need to cancel that order (whick by the way is way late in getting here)?

I indeed have a 3/4 ton.

No 3.55 gears for you then.

so my only option would be a tranny swap? This is my daily driver so I need just a little more from it.

You could do a rear axle swap, a Dana 60 from a Ford or Dodge van will fit, and are common with a 3.55 gear. Alternatively, if you can get lucky, there was a 4.10 ratio 3rd member used in the '68-'72 3/4 tons that is a direct swap into your existing differential.

Check out the Ton's o Fun section. There is currently a couple posts going on about swapping in a 14 bolt in place of the original axle. There's been a few guys that have swapped the rears in favor of 14 bolts or Dana rears. Of course there is going to be some fabrication involved.

yea I was following that thread whitedog thanks. Has anyone put in a M22 or have they mostly been t5s from camaros and such?

The T5, SM420, and sm318 seem to be the norm. Actually the T5's are mostly from S10's. The S10 had a mid mount shifter, whereas the Camero was to the rear. If you wanted to use the Camero transmission, you would have to convert it to the S10 style shifter or cut a hole in your seat.

Remember, going with a 3.55 gear is already like getting overdrive in a truck that originally had 4.57 gears.

The M22 wouldn't gain you anything, as the final drive is still 1:1, same as the standard transmission. The T5 would gain an overdrive gear, as would an MY6 4 speed.

I know everyone mentions taking a t5 out of an s10 but are there any companies that make a new tranny for this type of application? or is it that the s10s are easier to mate up to the current set up?

If you have an unlimited budget, you can buy a new T5.
http://www.tremec.com/performance/t5.html

why isnt there much mention about bolting the t5s to a 248? Also if the t5s arent great for a 3/4 ton then which trannies are?

The T5 is actually a pretty good transmission. The 4 cylinder versions, I'd stay away from, but the V-6 and V-8 ones are pretty good. It's biggest drawback is the Input bearing retainer wears out and the countershaft bearing retainer gives out. They let the shaft flex, and you guessed it, things break. Luckily, the aftermarket has fixed these problems. You can buy an iron input bearing retainer and a solid countershaft reatainer instead of a stamped steel one.

is the 248 straight six not a popular engine or why isnt it talked about much?

The 248 is a great engine and is very adequate for everyday driving of a pick up. However a 270 or 302 fits right in it's place and doesn't cost anymore to rebuild. Most people believe bigger is better so they go with the bigger engine at rebuild time.

The T5 is probably the most economical transmission to swap. Economical in that the transmissions don't cost too much, and give 5 forward gears. If you routinely haul heavy loads, or have a big hp big torque motor, the t5 isn't the best transmission.

As mentioned by Longbox above, the MY6 (A833) 4 speed is agood transmission to put in a 3/4 ton.

Complete overkill for a 248 would be a NV4500 5 speed. This transmission has a granny low and an OD, and will work with a combined vehicle weight of 14000 lbs or more.

I have a NV4500 in a 59 1/2 ton with v8, and another one ready to go into a 1940 gmc 3/4 ton with 302 6 cylinder. They are sweet transmissions, but on the pricier side as in $1400 or more to swap in used transmission, and about $2500-3000 for a new swap.

As far as a T5 and a 248... The 248 uses the same bellhousing as a chevy 235. Everything you have read about swapping a t5 in a chevy will cross directly over to the 248 gmc. In the case of your 1954, the transmission will almost bolt directly up to your bellhousing. I believe you might have to extend the splines on the input shaft, and hog out the holes on the mounting ears.

An MY6 (A833) swap will require some precision machine work to mate up to your bellhousing. Either the bellhousing has to be bored out to 5.125 inches, or the bearing retainer has to be machined down to 4.686 inches to fit in your bellhousing.