The Stovebolt Site Forums The Shop Area Driveline Mph difference from 3.73 to 3.08s

I just built a 51 3100.
Has a stock 1954 235.
1964 truck 3speed std transmission and a 1964 12 bolt truck rear end with 3.73s.
I don't have a tach so I'm not sure of the RPMs, but in 3rd at 40mph shes wound out, the tallest gear ratio I can find for that rear end is 3.08s (I'm afraid anything taller will lug the 235 anyway)
Does anyone have an idea of how many mph I will gain by swaping the 3.73s for 3.08s? I'd like to cruise 60 decently. But I don't see me gaining 25 mph with that swap.
Or maybe that transmission is geared funny and I could get a different 3 speed that would help the situation.

Maybe This gear speed calculator or That what speed do you want calculator will help you decide.

3.73/3.08 = 1.20 x 40mph = 48mph

I have a 3.55 rear end in my 1954 3100 trucks. With 31.5" diameter tires and 1:1 top gear, it can cruise nicely at 60-65mph (approximately 2300-2500 RPM).

The standard 3-speed transmission would be 1:1 in 3rd gear.

I pretty much have exactly what you are asking about.
'66 Saginaw 3 speed, 3.08 12 bolt rear end, stock diameter tires on 16" rims.
I can go 70 without blowing anything up.
I cannot get out of my own way in first gear on a steep hill.
No free lunch with that combo.
I am in the process of swapping out for a T-5 transmission with 0.86 to 1 overdrive, and a 3.73 rear end.
Carl

I have a 1957 3100. My 3 speed gave up third gear last year and I also wanted to cruise a little faster without running 6000 RPMs. I had already put a new ring/pinion in my rear (moved from 4.11 to 3.55 I believe)--that didn't make much difference unfortunately. What did make a huge difference is going to a Borg Warner T-5 with a 5th gear overdrive. I now cruise 70 mph at about 3200 RPMs.
There is a great article in an old copy of Classic Truck (don't have issue number for you) that I followed. I got a T-5 from a 1988 Camaro, and changed out the tail shaft with one from a T-5 from an S-10 so that the gear shifter location was just right for my bench seat. Toughest part was finding a clutch to work--article suggested a clutch for a Chevy Astro van that had a V-8 with a 5 speed transmission (there couldn't have been many of those). I went to Advance Auto Parts and asked for the above clutch and it worked perfectly. Best thing I ever did to help the driving ability of my old truck. To keep the truck looking original, I purchased a shifter handle from Lokar and it looks just like an old original 4 speed on the floor. Hope this helps.

CASO, thanks the chart says at 2500 rpm I should be doing close to 67 mph, that'd be great.
I thought about doing a t5, trying to make it to a show 100 miles away in a couple weeks and swaping gears will be alot easier then finding and doing a t5 swap.
Just don't want to cruise 40 mph the whole way. I appreciate the input guys. I'm sure I'll be going to a t5 within the next year or two.

Tclederman.....
So are you saying it should do about 48 mph at the top of third with 3.08s?
I can't be gaining 48 mph can I? I can't get that lucky..

"but in 3rd at 40mph shes wound out"

If your present top speed is 40mph (relatively slow) and you now have a 3.73 rear end: if you change to a 3.08 rear end you would increase your mph (at the same RPM) by 1.2 times (20% faster MPH at the same RPM).

40mph to 48mph at the same RPM

Oh Boy

I hope Tim mine isn't this slow....I wont wanna drive it...I have a 235/4 speed/ with a K5 Chevy blazer rear end in it...I am hoping for 50 mph minimum and be able to run out on the New York State Thruway and run along 55 to 60 mph and not worry about anything blowing up...I want to be able to go out to the Finger Lakes to see my daughter and not get run over in the process...it is about 4 hours from where I am at now....

I am hoping these numbers are attainable....

MikeC

MikeC,

I find it hard to believe that Dylan65's is that slow.

My calculations were based on his numbers.

Wow...it seems strange to me also that it's only running 40 mph with 3.73s.
But the speedo read 40 and my girlfriends speedo (driving behind me) said 40. And this isn't my first time building a car or driving something I shift by "ear", and at 40 it was running pretty hard. I will wire a tach up and see what RPMs I'm running.
I replaced the enclosed driveshaft 4.11 rear to avoid this situation.
Thanks for the help. I'll let y'all know how it goes.

Dylan, If you don't have a tach, how do you know that it is wound out at 40? Any 3 speed you get will be 1 to 1 in 3rd gear. They would run 70 all day long with 4.11 and 3.90 back in their day. It will certainly run 70 with 3.73. Is the noise getting so loud that it scares you? If so, thats the way they are. Get used to it. If the motor won,t run any faster, you have it timed too late, or you are not getting enough gas. 3.08 will cause a 235 to be a real dog at the low end.

Well I don't need a tach to tell me the motor is screaming, but I'm going to wire one up and see what it's running. I've had a truck with a 216 and a car with a 235, I'm used to 6s. I'll also check the timing and I'm sure it's getting plenty of fuel.
I pulled the engine out of a friends 54 a few months ago and didnt change a thing, just set in my truck. And It was running great in his truck.

I dont see how you can possibly be wound out at 40mph. I have a 3.73 with 15" 235/r60 tires with a 3 speed now and i can do 60mph at 3200rpm i can get it up to 4000 and then she is ready to jump out of the engine bay.

Im doing a t5 rebuild and swap right now. the first t-5 i got did not have the best ratios so I found another one better suited. the second one I bought has the 3.78 1st and the .72 OD. For now im going to run that with the 3.73 rear just for a bit then might switch the rear to a 3.42 mid summer.

The .72 OD with the 3.73 rear will take the 60mph at 3200rpm ish and lower that to about 2250 ish

would like to find a 3.50 or 3.35 first ratio trans then i could keep the 3.73 rear and not have a granny gear issue when starting out

I'm running a 235/75-15. 28" tall tire. This is why I am a little confused on why she doesn't wanna get over 40. I can't afford to do a t5 right now, but eventually I will.

will This calculator help?

Are you positive that you have a 3.73 gear?
A 235 with your 28" tires with 4.11 stock gears would not start screaming until 55 MPH, and would continue to go faster than that if you dared.
Maybe your 235 is screaming at you for other reasons than gear ratio.
Carl

i second what carl says about something else being wrong.

Without actually rotating the wheels and counting the turns of the drive shaft all you 'really' know is what the outside of the box says... Can you really trust that cold piece of steel not to be lieing through it's teeth?
Some fool hotrodder may have swapped something out years ago and neglected to engrave the changes... for lack of an engraving tool or just plain being in a hurry.

With your 3.73 ratio and tires size you should be at 1800 ish rpm when doing 40mph.

I found the problem, like an idiot I was looking over little stuff, the throttle linkage was hanging on the shaft it swivels on (on the side of the block close to the exhaust outlet) and I was only getting about half throttle out of it. So I wasn't running many RPMs, I guess with straights, and no insulation in the cab yet and being "floor boarded" I thought I was running alot more RPMs.
I fixed it from hanging up and cleaned out the blow by tube and it ran 60 pretty decently.
I appreciate y'all's help and input.
I did the tire trick and got about 3-3/4 turns of the shaft, but I also ran the numbers on the ring gear and they're darn sure 3.73s. And now it's acting like it should.
1 more question, is a 54 truck 235 solid lift?
I think the lifters need to be adjusted, shes got some clatter, or is it just natural for them?

Should be solid lifters. They have some clearance, so you will have some noise, but only what sounds like twelve sewing machines. Without that noise, you wouldn't know it was still running at stop signs.

Awesome. It's running good and I'm happy with it. Thanks for y'all's input.