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Old 01-13-2022, 11:55 AM   #1
James Lamb
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Cummin 12 valve Square Body Suburban MPG?

I have a 1989 Suburban 1/2 ton 4 x 4. 700R4, 3.42 diff. To save a long story, it originally was powered by a NA 6.2 diesel (had 2 of them) then by a 6.5 Diesel with Holset turbo. All of those engines failed.

My question is what kind of real world mpg does a square body get with a 12 valve Cummins? I've already done all the math about the cost and the length of time/number of miles to recover the expense, so please, I am just interested in real world fuel economy from someone who has done this.

For informational purposes, this truck is a working 4 x 4 - I'm a paleontologist, so it gets used as a four-wheel-drive. But basically only in mud, no rock crawling. It spends 99% of the time on secondary state highways getting to and from one site to the next, at between 55-60 mph, almost never on the interstate. So I'm about to go back to P-metric all-terrain tires from the LT tires it has now. I expect to pick up a little efficiency there. It's also about to get a 4L60e with aftermarket controller. Same final ratio as the 700R4 - why is another long story.

Driving it in my current use with the anemic 6.2 I got 22-23 mpg. With the current engine, a 383 with rv cam and a quadrajet I get 14.5.

I was thinking about doing some things to improve mpg and drivability such as an aftermarket EFI system, but there is a chance we may end up moving to a state with vehicle inspection, in which case I need to consider diesel options before I sink more money into a gas engine set-up that would be wasted money if I can't drive it. Otherwise I would leave the 383 in it and be happy with adding EFI.

Also, anyone have data on other diesel engines? The VM Motori does not have good longevity from what I have read. 3.0 Duramax costs $$$$$ even for a used one. Not sure a Cummins 4BT has the juice to move a vehicle as heavy as this efficiently.

Thoughts?
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Old 01-13-2022, 02:26 PM   #2
Dieselwrencher
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Re: Cummin 12 valve Square Body Suburban MPG?

My P-pump 24v 5.9 Cummins in my 72 Longhorn gets 16.5 in town. The truck weighs 7200lbs with me in it and it has huge injectors and 2 turbos. Driving normal with some skinny pedal action, it gets 16.5. Highway it gets 18-20. Most swaps I've done a 12v will get 18 plus. I did a yellow square body 4x4 burb that was on 35s, had 3.54s and it got 18 or better on the highway. It got 12mpg pulling a 10k dump trailer loaded with rock at interstate speed.
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1972 Chevy Longhorn K30 Cheyenne Super, 359 Inline 6 cylinder, Auto Trans, Tilt, Diesel Tach/Vach, Buckets, Rare Rear 4-link and air ride option Build Thread
1972 GMC Sierra Grande Longhorn 4x4
1972 Chevy Cheyenne Super K20 Long Step side tilt, tach, tow hooks, AC, 350 4 speed
1972 C10 Suburban Custom Deluxe
1969 Chevy milk truck
1971 Camaro RS 5.3 BTR STG3 Cam Super T10
1940 Ford 354 Hemi 46RH Ford 9" on air ride huge project


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Old 01-13-2022, 02:40 PM   #3
James Lamb
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Re: Cummin 12 valve Square Body Suburban MPG?

What sort of speeds are typical for you with that mpg?
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Old 01-13-2022, 08:27 PM   #4
Dieselwrencher
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Re: Cummin 12 valve Square Body Suburban MPG?

70-75mph, even 80 if you have 3.54s
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1972 Chevy Longhorn K30 Cheyenne Super, 359 Inline 6 cylinder, Auto Trans, Tilt, Diesel Tach/Vach, Buckets, Rare Rear 4-link and air ride option Build Thread
1972 GMC Sierra Grande Longhorn 4x4
1972 Chevy Cheyenne Super K20 Long Step side tilt, tach, tow hooks, AC, 350 4 speed
1972 C10 Suburban Custom Deluxe
1969 Chevy milk truck
1971 Camaro RS 5.3 BTR STG3 Cam Super T10
1940 Ford 354 Hemi 46RH Ford 9" on air ride huge project


Tired of spark plugs? Check this out.
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Old 01-13-2022, 08:36 PM   #5
James Lamb
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Re: Cummin 12 valve Square Body Suburban MPG?

Ah. That makes sense then. I got 17 with a 6.5 turbo, and I always thought of the 6bt as more fuel efficient. But I don't typically drive as fast.

Thanks for the info.
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Old 01-14-2022, 06:58 AM   #6
Dieselwrencher
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Re: Cummin 12 valve Square Body Suburban MPG?

Quote:
Originally Posted by James Lamb View Post
Ah. That makes sense then. I got 17 with a 6.5 turbo, and I always thought of the 6bt as more fuel efficient. But I don't typically drive as fast.

Thanks for the info.
Some 6bt's are more efficient than others. The VE rotary pump engines do really well. I had one I put in a 95 F350 crew cab long bed 4x4 that would get 22 highway and 18 pulling 18k no BS. I never should have sold that thing.
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1972 Chevy Longhorn K30 Cheyenne Super, 359 Inline 6 cylinder, Auto Trans, Tilt, Diesel Tach/Vach, Buckets, Rare Rear 4-link and air ride option Build Thread
1972 GMC Sierra Grande Longhorn 4x4
1972 Chevy Cheyenne Super K20 Long Step side tilt, tach, tow hooks, AC, 350 4 speed
1972 C10 Suburban Custom Deluxe
1969 Chevy milk truck
1971 Camaro RS 5.3 BTR STG3 Cam Super T10
1940 Ford 354 Hemi 46RH Ford 9" on air ride huge project


Tired of spark plugs? Check this out.
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Old 01-15-2022, 06:47 PM   #7
Corts60
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Re: Cummin 12 valve Square Body Suburban MPG?

The 80’s gm diesels were made to be fuel misers so they always get the best mpg. My 5.7 consistently got 22-23 mpg at 65/70. Any faster than that and it dropped down to 18/19. Just can’t pull anything serious. My buddies 6bt in a 4x4 dodge got 16mpg all day long. I used to drive a 7.3 for a short minute and it got 13-14mpg loaded or empty.
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Old 01-15-2022, 08:03 PM   #8
v30crewcab
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Re: Cummin 12 valve Square Body Suburban MPG?

I get 18-19 out of my crewcab on 35's.
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