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?