Cummin 12 valve Square Body Suburban MPG?

[James Lamb]

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?

[Dieselwrencher]

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.

[James Lamb]

What sort of speeds are typical for you with that mpg?

[Dieselwrencher]

70-75mph, even 80 if you have 3.54s

[James Lamb]

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.

[Dieselwrencher]

Originally Posted by James Lamb

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.

[Corts60]

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.

[v30crewcab]

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