Highest MPG V8?

[theholycow]

The Quadrajet, if it's not the most efficient, is one of the most efficient carburetors ever to hit the mass market.

I always support efforts to improve fuel economy in any vehicle and try not to be a naysayer but I don't think it is realistic to look for 20+ highway in a 1-ton dually 1985 model. I doubt you could manage it even with a modern small turbo diesel and a manual transmission, let alone a carbureted 350 gasser and an automatic. There are probably things you can do to improve it but I'd be surprised (and happy!) to see you hit 20.

Since you're asking about gears, how's highway RPM and power? If it's already downshifting to get up hills then it's probably as tall-geared as it can use.