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Can I swap out rear end gears on my stock 54 3100/ 3:90:1 for something lower to go faster than 50 mph???? I have the hydra-matic automatic transmission!

There's a 3.55 gearset available for your differential from Patrick's.

You'll also need a 1953 or earlier (back to 1940, I think) 1/2 ton differential carrier when installing the 3.55 ring/pinion gears from Patrick's in a 1954 truck. Also, consider buying the bushing/bearing/etc installation kit from Patrick's (almost doubles the cost but it is money well spent).

Before you start, are you aware that changing the ratio from 3.90:1 to 3.55:1 is certainly better, but if your top speed before was 50, it's now 55? That's all it does.
Is that what you want?

I cruise easily, economically, safely, and quietly between 60-65 mph (with the 3.55 gear set).

1shot,
You should be able to easily do 60 plus mph in your truck if the engine is in good shape.
I've spent time behind the wheel of a '56 and a '59 half ton truck, both with 235 and 3.90 rear gears and could cruise pretty comfortably at about 60-63 mph. It is not quiet though...some guys don't like to wind 'em that high.

I'm sure glad nobody told my '58 Biscayne with the 14" wheels and the 235 that it couldn't run faster than 55 with a 3.55 gear! It ran anywhere from 75 to 90 without much effort!
Jerry

Reading comprehension problem, have we?
That's not what I said - it's what he said.
Exactly what part of my statement is incorrect?

THANKS EVERYONE>>>>>My truck does 55mph, but right then it makes a horrible sound if i push it any harder, so i always back down. I wont do the rear end if it only get me 5mph, If i can get 10mph i will do it for sure. Everthing else runs great thats why i'm keeping stock drivetrain. I just need it to run smooth at 55, not winding high and not sounding good! So mabye something else is wrong but every one i;ve talked to says 55mph is about it. My manual says it only has a max speed of 63mph.

Just a thought, could there be a possibilty of a clutch slipping in the trans? I do know that there are some adjustments on the early Hydo trans, prehaps some service is required. Jerry would know much more about this that I do.

If i can get 10mph i will do it for sure

And you can, but not with that gear change.
This is not covered by opinion, or experience, it's math.
3.90 ÷ 3.55 = 1.10141
If 50 was the highest safe speed before, and you change nothing else, the new safe speed is going to, exactly, 1.10141 × 50, or 55.070.
To get a different answer, you must do 1 of 2 things:
1. change something else
2. be wrong

Here, you do the math...

edit: Not disagreeing with anyone, I didn't see if he posted a tire/wheel combination.

As you can see from my byline below I achieved my cruising highway objective by dumping the closed driveshaft, adding the '57 3.90 with a '62 3 sp+od, that's .7 X 3.90 = 2.73. Initially, actually, I mean before I added the above combination I added the "poor man's overdrive" read: taller set of tires which lowers rpms.

Thanks......I'm running 235/75/15's right now. Is it possible its a timing issue, or driveshaft adjustment, trans adjustment? I'm new at the truck thing so any info helps!!! Thanks!

Growing up, my Dad had a 1960 Carryall with a 235 and 3.9 diff. Back then the speed limit was 70 on all our freeways. He put 300,000 miles on that truck, a lot at 70 MPH.

. . . but every one i;ve talked to says 55mph is about it . .

They are wrong.

My larger diameter radial truck tires (on stock 16" rims) get me about 9% better speed at same RPM compared to the 15" tires you have. Add the approximately 10% improvement from the 3.55 conversion and you are close to a 20% improvement.

I can (and have) cruised all day at 65 mph. Two people can talk at that speed at normal volume.

If you get funny noises now at 55 mph, you have problems somewhere (u-joint, transmission, rear-end, wheel and/or axle bearings)?

From your description, I think something is wrong - driveshaft, wheel bearing, damper, etc. and it should be fixed before you do anything else.
A taller tire has exactly the same effect on speed as a ratio change: new ÷ old = change.
Replacing a 27" tire with a 28" tire increases speed by 28 ÷ 27 = 1.037.
50 mph become 51.85 mph.
You can't get 10 mph out of a tire change.

Follow this link and open the excel file. This came from the oldgmctruck web site and is helpful.

http://www.oldgmctrucks.com/SPEED_RPM_CALCULATOR.xls

Thomas

I think some of us are too accustomed to modern, quiet vehicles with their hydraulic valve lifters, smoother riding suspensions and sound deadening insulation. Old Chevy trucks are loud, and were when they were new. Lots more mechanical noise, especially from mechanical lifters and road noise. I think that is part of the reason we think it's about to fly into pieces when we get to about 50, so we back down. In my 50 3600 with its 216, 4.57 rear and 7.00-15 tires the motor's turning approx 2850 RPM at 55 MPH. I drove most of 330 miles last May at that speed and it ran great. But my ears were ringing when I got there.

well the noise i got a 53mph was my cooling fan rubbing on the pully, it must of flexed enough a high speeds that it rubs and makes a horrible noise. Few washers, temporary fix, and 65mph i go.