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Old 12-21-2021, 05:12 AM   #1
evan love
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Smile 454 Dually with 4.56 rear diff, what auto trans to use

hello,

I just bought a truck I've been looking at for a long time and finally knocked on the door and they sold it to me for a great price.

It is a 1979 single cab dually (dooley) 2wd with a 454 and automatic (almost no rust and good original paint, Silverado with great options)

The only issue is it has a 4.56 rear diff, which can pull just about anything, just not at highway speed.

It also has the 10,000 GVW and the camper package.

What I would like to use it for is to tow a flatbed car trailer with a 1973 K10 shortbed on the trailer, and I would like to drive it from Colorado to California.

Which gets to my question...

... do I switch the rear diff gears and lose some pulling ability, or change the 3 speed automatic for a 4 speed automatic, or something else so that I can actually still have the power to tow five or six thousand pounds and still get down the highway at an ok speed and minimize gas usage and engine wear as much as possible?

thanks,

I am open to any and all options with a mind towards low to superlow budget combined with the fact that I do have mechanical skill and tools, help, and access to a lift (though not sure the dually will fit on it)

thanks again,

evan
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Old 12-21-2021, 09:58 AM   #2
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

If your ‘super low’ budget is 3 to 5 thousand bucks then you’re good.
That’s what it will cost for a trans strong enough to do what you need.
Or a gear vendors overdrive.

What size are your back tires?
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Old 12-21-2021, 11:29 AM   #3
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

A gear vendors is just as expensive as a 4L80e. If you go new. Rather you buy used new or rebuilt.. Plus it has a 20% overdrive vs a 30% of the 4L80e.

Plus it would not have a lock up converter which I would want if I wanted a real tow pig. So my choice would be a 4L80e.

https://www.car-part.com/mobile/index.htm

Is a good resource to help find parts, and often cheaper then ebay.

Forgot to add. If you switch gears to get good highway manners. You loose to much pulling power. The 4.56's are to me perfect with a 4L80e, and 30" tires. You will be about 2500 rpmat 75, and have enough air flow over the carb to get optimal mpg.
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Old 12-21-2021, 12:47 PM   #4
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Tire size is going to be important here---and ---are there any mods to the engine itself
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Old 12-21-2021, 02:31 PM   #5
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Gear swap is a tradeoff.

You have a BBC in a Squarebody truck. Erase the notion of "gas mileage" from your vocabulary.

4l80 or NV4500 swap. Best of both worlds. Pick your poison. I been there done that.

I LOVE my 4:56 gears behind my 4500. Buy once-cry once.
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Old 12-21-2021, 03:23 PM   #6
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

4.56 gearing in my suburban, 285/75r-16s with a convert to a 4l80e. Runs sweet
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Old 12-21-2021, 07:29 PM   #7
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Considering I've towed a square body with another square body using 3.08 rear gears with a vortec 350, I think you'll do just fine re-gearing. That being said, I'd re-gear [to 3.73 or so depending on wheel diameter] AND stick an overdrive in there. But 4L80 is the only automatic trans option in a 1 ton, and its expensive plus requires a stand alone computer controller that cost a metric crap ton more then it should.
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Old 12-22-2021, 09:28 PM   #8
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

i tow with a GMT-400 1.5 ton 3500-hd truck . its a 8.1 bbc vortec and nv4500 combo with 4.63 rear gears . the trailer i pull every day is 10k loaded full .

i would not go lower if you have any hills . if its all flat land and no hills to climb maybe go better gearing . but its a work truck let it eat and just work .

my truck was ether 4.63 or 5.13 gear . i live in a lot of hills and long climbs and wish i had the 5.13 for that stuff but the 4.63 does good so i just run it .

make sure tire height is correct and bigger = lower rpm but more work for pulling loads . and changing gears = lots of money and unless BIG change its only a few 100 rpm change per jump .

if what you have is working but want more i say go bolt on over drive to let you split each gear to make it a 6 speed . but yes lockup conv on a 4L80-e is nice . but if pulling BIG / HEAVY loads do NOT pull in O/D as it will kill the clutch pack and clutch in the conv faster . years ago i had a truck with a vac guage and it got better running performance in 3rd than overdrive when pulling a loaded trailer . it was a vortec 350/4L80-e / 4.10 combo .

good luck and plan correctly as most changes take for ever to pay you back unless you use it that way every day .
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Old 12-29-2021, 05:08 AM   #9
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Thanks for all the replies.

I guess I didn't know where to look to see this thread again, I actually thought it just didn't work so I kinda made a new thread too.

Ya, it seems like getting the money back in gas mileage might be so far out in the future as to never actually arrive.



Any thoughts on just running kinda high rpm's on stock rebuilt 1979 454 with an Edelbrock carb (not sure about cfm)... ... what rpm could it handle for hours on the highway (Colorado to California)?



I guess the only really low budget option is rear diff gears and taller tires when I need tires again.

How much towing capability would I lose going from 4.56 to 4.10 or 3.73?

thanks ya'll,

I appreciate the input
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Old 12-29-2021, 08:47 AM   #10
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Id think you could go with the 4L80 for less than $3k. The controllers are around $500, trans is all how good you are at finding stuff. Local yard has em for $350. Duno what t-case you have, but there is info out there to adapt to any t-case. This way, you still have the power to pull whatever, and be able to cruise on the highway over 55mph.
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Old 12-31-2021, 07:55 AM   #11
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

well, it's 2 wheel drive so no transfer case

if i go 4l80e i guess i'd need the controller situation and an appropriate torque converter...

...would I also need a different shifter? It's column shift now


thanks,

evan


oh, ya, what size tires are you guys using on a dually with stock steel rims?
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Old 12-31-2021, 06:28 PM   #12
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

My 81 C30 has 4.10s and a nv4500 and 31in tire..I just pulled a travelall on a 16ft trailer from Colorado to Alabama..most of the time I had to look in the mirror to even know it was still behind me..except in the high pass elevation..it was a dog ...35-40mph was all I could do..other than that ,I could run 75-85mph with no issue..my LQ9 averaged right at 13mpgs..
In Alabama I regularly pull around 9500lb gross loaded on trailer..very seldom even feel it..
I think youd be fine with a od trans
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Old 01-03-2022, 12:53 PM   #13
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

This is a bit of a no win situation.
That truck was designed/built in carburetor times and pre-overdrive too. The thing you have to realize (if you are not old enough to remember) is that speed limits were lower back then.
In 1974 the government shoved this down the throat of the states by withholding Federal Tax dollars to the states to force them into it.
The original intent was to reduce fuel consumption. Then the propaganda of "Safety" was used to reinforce the narrative.
The auto manufacturers started working on both. This gave us fuel injection and OD transmissions, which improved mileage and emissions at the same time.
The safety bit gave us anti-lock brakes and airbags. These came along over several years of testing and improvement, becoming essentially standard.

Now we have lane-departure warnings, smart cruise control, etc. Self-driving cars are on the horizon, I imagine with the goal of fully connected cars that cannot crash into each other. (Probably with the ability to actually drive them yourself, but still override your input to avoid crashing) I'm thinking that the ultimate goal is that you would have to enter a destination into the connected car, which helps it even more, since some centralized controller would then know where all of the cars are going.

Sorry, ran off the rails a bit there ^^^
All that to say Big Block, carburetor, gear to pull = lower fuel mileage and reduced highway speeds.
In today's world, you are literally messing with an antique. There are some workaround's, but they are likely to be more costly than valuable.
A smaller, more fuel efficient engine, with a compatible OD and higher gears can help quite a bit. The lower 1st gear of an OD will compensate for the pulling power and the higher final drive ratio will bring the RPM down on the highway at the same time. But, that is effectively a new truck stuffed into an old shell. Do-able, even more-so if you do all of the work yourself, then you end up with a cool old truck in the long run, but at what cost?
Superlow budget.....nope.
Swapping to a 3.73 gear is the economical attempt and probably worth a try, but it's not going to get you all the way there. Even this is a $500 option, doing it yourself, which I generally do not recommend, unless you have done it before.
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Old 01-04-2022, 08:47 PM   #14
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Hi Longhair,

Well, I guess it'll be diff gears first, then 4l80e when I get to it...

...in the meantime, at least I got to see your David Allan Coe reference
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Old 02-02-2022, 01:08 PM   #15
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Was just searching for a similar thread before I made my own.

Im not planning on actually towing/working the truck at all, and I dont really care about gas- I'd just like to be able to do highway speeds around here (70-75) without sounding like the 454 is gonna blow.

Not sure of my rear end gears (no glove box code) but its a 1979 GMC crew cab dually camper special with the 454 and 3 speed auto.

I assume changing my gear ratio is the best way to go to get more more highway comfort?
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Old 02-02-2022, 01:30 PM   #16
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

A gear swap from 4.56 to 4.10 will yield you a 10.1% difference in RPM. So spinning the BB at highway speed of say 2800 will drop you to 2520. An overdrive swap to a 4l80e (25% reduction) will give you about 2100 rpm or effectively drop your rear diff ratio to 3.42. Low 2000s in rpm for a big block is the sweet spot for cruising.
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Old 02-03-2022, 11:24 AM   #17
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

Quote:
Originally Posted by kwmech View Post
A gear swap from 4.56 to 4.10 will yield you a 10.1% difference in RPM. So spinning the BB at highway speed of say 2800 will drop you to 2520. An overdrive swap to a 4l80e (25% reduction) will give you about 2100 rpm or effectively drop your rear diff ratio to 3.42. Low 2000s in rpm for a big block is the sweet spot for cruising.
would 3.73 be worth the swap? seems cheaper and easier than swapping the trans?
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Old 02-09-2022, 03:35 PM   #18
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

My '90 K30 3+3 454 got the best mileage @ 55 because of no overdrive (and made a big difference), a 4L80 would've been ideal for it.
I don't believe the minor difference of changing from 4.10 to 3.73 is cost effective - EVER ! (unless you drive coast to coast frequently)
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Old 02-09-2022, 05:21 PM   #19
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Re: 454 Dually with 4.56 rear diff, what auto trans to use

4.10s with an overdrive is the sweet spot, but it is pricey. Have that set-up in my dually flatbed. I have a T400 with a gear vendor in that truck. My pick-up has a Ranger OD. And my Suburban was converted to a 4L80
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