I had a '79 250 with a 1 barrel, and it got about 14 city, 17 or so highway. It ran great, though. I was going to put a larger carburetor and intake on thinking it would help the mileage and power, but ended up trading it for a bigger truck. 500 CFM or so 4 barrel and a new intake would probably improve mileage a few mpg's, and definitely the power. - Mr Diesel
'76 C30 Crew 6.2