[James Lamb]

I have a 1989 Suburban 1500 4 x 4. It was originally a diesel, got tired of replacing engines, built a 350 with a quadrajet for it. Went back as stock as possible, GM pre-vortex heads, mid-range cam like you would find in an RV to keep the torque down where I wanted it for 4 x 4 work.
Long story, drove it a couple thousand miles, thought there was a problem with the engine and took it back out. No problem, as it turned out, but I decided to rebuild it as a 383. Unbolted the intake, with carb attached, set it aside. Bought a new 383 internal rotating assembly for $1000, reassembled engine, bolted intake/carb back. The exact same block and everything except the 383 specific parts, no adjustment to carb. I gained 1 mpg driving it on exactly the same drive I tested the 350 on, which is average speed 55-60 mph, some flat, some mildly rolling hills.

My take is the extra fuel economy is due to the extra torque making the transmission plenty happy staying in overdrive at any speed over 45 mph, and the engine not working hard at all moving the truck down the road. The extra hp and torque is very noticeable, I have a hard time finding a hill steep enough where I live to make it shift out of overdrive.

I run tires 2 inches bigger diameter than stock, which lowers engine rpm and there is plenty of room under the fender for taller tires and a "free" 1inch lift. Have 3.42 gears, built up 4l60e with aftermarket controller, have a speedo conversion that corrects the speedometer/odometer.

I finally got sick of the quadrajet, installed a Holley Sniper with Hyperspark distributor, so the ecm controls fuel and timing. Runs much much smoother, starts easy, don't have mpg figures yet for it. I feel like it gave me a little bit of extra hp, but I have no quantification of that other than seat of the pants.

With the 350 - 13.5 mpg average. With 383 14.5 average. I also have pretty aggressive tires, so could probably bump that some more with more road-like tread. Don't know that I will ever recoup the cost of the Sniper system, but I was after reliability and driving convenience.

I don't know what a 383 rotating assembly costs now, but not many mods can be done for less than $1000 with any expectation of increased hp/mpg. When I first bought the truck, more than 20 years ago, I replaced all the wheel bearings, went to synthetic fluids throughout the entire drive train. That got me just a tiny bit shy of an extra 1 mpg, averaged before and after over 1000 miles of driving.

Good luck with the project.