Will I regret change to 3.55s from 2.76s

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1973dust

1973dust

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I'm looking for some experienced opinions as I've searched thoroughly on here and I still can't decide. My car is a 73 Duster with a stock bottom end, stock head 74 318. Engine mods are Performer intake, 600 Edelbrock carb., cheap Summit cam 204/214 278/288 .421 .444 112LS, decent ignition curve, and duals off of the stock manifolds. Drivetrain is stock stall 904 and 2.76 peg-leg 7 1/4 with 27" tall rear tires. The trans. does have the part-throttle downshift valve body and I have modified the kickdown linkage to give me 2nd gear out of low-speed turns with fairly light throttle. I seldom really run my car hard but would say that I always accelerate "briskly". I definitely don't ever hold up traffic. I feel like the car runs really well the way I have it set up now and I get 15-16 mpg with a mix of in-town and 55mph 2-lane roads. I'm sure it would get 18-19 on an extended highway run. I have a 3.55 geared peg-leg 8 1/4" that came from a Jeep Cherokee that I was going to install but I'm starting to wonder if it will be a mistake. Obviously, I'll have to remove the Jeep perches and weld new ones on, shorten a driveshaft, and swap over the brakes. The wider Cherokee rear would fit perfectly with the rims I have as I'm currently using 1 1/2" bolt-on spacers on the 7 1/4. If I had a 8 3/4 where I could just swap the center section I would just try it and see. Certainly, my car would be quicker with the 3.55's but I'm not sure how much gas mileage I'll be sacrificing although I suspect 2-3 mpg. I'm currently running about 2100 rpm at 55 which would increase to about 2700. I do realize that the engine would not have to labor as hard to get up to speed with the deeper gear but I also would be deeper into the throttle. If you've taken the time to read this novel, thanks for any thoughts you can offer.
 
What is the fastest speed you want to drive long range on the freeway?

Did you change valve springs?

A gear change will really make it a different car.
 
How much do you drive it? Do you want to make long trips?

Here is my story.....
I pulled the 4.10 from my dulley ram to install a 3.56.
Picked up 2 MPG around town with no load at less that 50 MPH.
But towing my trailer at 6K lbs it became a DOG.
If you look at Torque RPM Chart I moved the engine into a shitty point on the curve.
So I put the 4.10 back in.

Think about that.
 
You need 3:55's with 2:76 gears and a 27 inch tire. With the tall tire's it will be more like a 3:21-just right
 
413 said:
What is the fastest speed you want to drive long range on the freeway?

Did you change valve springs?

A gear change will really make it a different car.
Click to expand...

Hyper_pak said:
How much do you drive it? Do you want to make long trips?

Here is my story.....
I pulled the 4.10 from my dulley ram to install a 3.56.
Picked up 2 MPG around town with no load at less that 50 MPH.
But towing my trailer at 6K lbs it became a DOG.
If you look at Torque RPM Chart I moved the engine into a shitty point on the curve.
So I put the 4.10 back in.

Think about that.
Click to expand...
I'm never on a road with a speed limit higher than 55 and never drive over 60-65. I did not change the valve springs yet but I have Comp 901-16's new in the box. I normally drive it 1500-2500 miles a year in nice weather and don't have any intentions of any long trips. Obviously, the hotter than stock cam doesn't make much sense if I leave the 2.76s. While answering you guys questions, the 3.55's seem to be becoming more of a no-brainer.
 
Murray said:
You don't know "briskly" until you put in 3.55. If you are concerned about gas mileage, stick with the 2.76.
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Gas mileage isn't the most important factor but any guess what kind of drop I'd be looking at?
 
TF360 said:
You need 3:55's with 2:76 gears and a 27 inch tire. With the tall tire's it will be more like a 3:21-just right
Click to expand...
I've also thought that if I shimmed the front spring hangers back 1/4-3/8", I could probably fit 28's if I wanted to drop the cruise rpm some.
 
Went from a 2.76 to a 3.23 in my 360/727 69 barracuda. It definitely has a lot more pep. The only negative is that it winds a little high for me on the highway. About 2800 at about 60 mph. If you don’t drive long distances you will probably like it a lot.
 
to be honest only you can know if you will hate them or not. in my old dart i went from a 3.91 to a 3.55 and it was better but i still didn't like it... too much gear for my driving type..

our present dart has a mild 360 in it and i originally had a 2.94 gear in it, then wnt to whats in it now a 3.23 gear.. while it made a 5,000 mile trip last year with no issues my plan is to throw a set of 2.76 gears in it this year.. it will cruise better for what i want to do with it. i was really on the fence with what gear i wanted the 2.94 or 2.76 playing around with a rpm calculator told me i wanted the 2.76.


here play around here and see what RPM you'll be spinning and if you'll be ok with it.. Speed/Tire/RPM calculator






.
 
3.55 might be more than you want.
3.23 is still a lot better than that 2.76 you have!

I am going to run a 3.23 in my 76 Dart Lite.
4 speed OD box.
 
Hyper_pak said:
3.55 might be more than you want.
3.23 is still a lot better than that 2.76 you have!

I am going to run a 3.23 in my 76 Dart Lite.
4 speed OD box.
Click to expand...
That is exactly my concern. I was actually hoping the rear was going to be a 3.08 when I bought it as I think that would be a good compromise. Although, I just did what should have been some obvious math. If I drive 2000 miles in a season, getting 14mpg instead of 16, that's only an extra $72 even if gas is back at $4 a gallon.
 
I would use the calculator and play with the numbers.
But nothing will replace you using it and making the decision best ffor you.
My 4.10 to 3.5 change was an expensive learning experience.
 
1973dust said:
Is that only in a good way? Part of my problem is that I've never had a Mopar with anything deeper than 2.76's.
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It will perform much better, so much you won't believe it's the same car. 2.76 really kills the torque.
Nobody can tell you the change in mileage, you change the gears and then tell us.

For an extra $72 a year in gas just go for it. Since you have 27" tires then go for the swap. If you had 25" tires it would be too much gear for freeway.
 
I had a 69 Fury ragtop, stock 318 long block except for the 340 cam, and ran a six pack. Originally built the car with 3.23 gears and swapped in 3.55s a few years after. 225/70/15 tires. If I drove it like a normal person, I could get 18 mpg on the long road and roughly 14 putting around town. With the 3.23s I was at 19 and 16. The seat of the pants difference was worth every extra gallon.
 
I had 2.45's to begin with. Total garbage gears!! Then went 3.55's wow what a difference! Then finally got greedy and went 3.91's and hold on much nicer. I'm keeping my 3.91's!!!:steering:
 
I have a '72 Duster here, a project that my brother in law abandoned....

IMG_9824.JPG


When we first put it together, (2007...2008) it had a stock 360 from a van, a 340 cam and Holley 600. 4 speed 833 and 3.91 gears. For a fairly stock engine, the dude really ripped!
Jeff decided he'd rather have an automatic so I converted it to a floor shifted 904.
It still hauled ***.
Then the car sat for 10-12-14 years until I bought him out in 2022. Over the years of it sitting I pulled the 3.91 gear out and stuffed a throwaway 2.76 in it. I recently got it running again and it does run great.
Guess what? Even with the 2.76, the car does alright. The longer stroke of the 360 was still able to get the car moving pretty good even without the deep gearing. I may stuff a 3.23 in it just to pep it up a bit. This is destined to be my 13 year old nephews car:

IMG_3919.JPG
 
Murray said:
You don't know "briskly" until you put in 3.55. If you are concerned about gas mileage, stick with the 2.76.
Click to expand...
I don't want this to sound bad, but I don't understand people who get into this hobby, have a nice car with a lot of money invested and then worry about gas mileage. I realize the Op in no way indicated that he was worried about gas mileage. I'm not referring to him. I get about 10 MPG in town and about 14-15 on the highway with my moderately built 340 4 speed and 3.55 gears. I am fine with that. I look at it as 10 Smiles Per Gallon in town Now if the mileage went down to 5 MPG in town, I would worry about what the problem was that caused a drastic reduction in MPG and fix it.
 
Absolutely. How many dudes drive these cars as daily commuter cars?
I used to drive as much as 1000 miles a week to some out of town jobs. I drove a 2007 Dodge truck that got 16-17 on the freeway. I was fine with that.
However....One advantage with improved gas mileage is that it allows you more range between gas station stops. If you just drive the car around your town, go for what makes it the most fun, whatever your definition of FUN is.
 
harrisonm said:
I don't want this to sound bad, but I don't understand people who get into this hobby, have a nice car with a lot of money invested and then worry about gas mileage. I realize the Op in no way indicated that he was worried about gas mileage. I'm not referring to him. I get about 10 MPG in town and about 14-15 on the highway with my moderately built 340 4 speed and 3.55 gears. I am fine with that. I look at it as 10 Smiles Per Gallon in town Now if the mileage went down to 5 MPG in town, I would worry about what the problem was that caused a drastic reduction in MPG and fix it.
Click to expand...
the same ones who worry about having to run premium gas.

that whole $5 more per tank is a total deal killer!
 
First of all...don't over think it.
Second of all, if it ends up not suiting you, you can change it back.
By your posts, you like to accelerate from a stop briskly, and the 3:55s will put a smile on your face compared to the 2:76s.
And gas mileage isn't a huge deal to you anyway.
I say go for it and try it, what have you got to lose?
Maybe a little bit of time and a few bucks, but that's what it takes to enjoy our cars...

BTW, I have never worried about gas mileage on my old cars anyway, if it is decent, then that's a bonus!
 
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