[aerohead]

I have a '62 Dodge D-100 which got 11-mpg stock,with 4:56 axle and 3-spd.
I swapped a 4-speed OD and 3.50:1 axle out of a '77 Dodge and this pushed the mpg (hwy) to 17 mpg.
A softened nose extension with integral airdam and minimized grille inlet,electric cooling fan,partial front wheel gap-fillers,full rear skirts,steel radials,full wheel covers,and aeroshell raised it to 21 mpg at 65 mpg.I tried higher and lower speeds and the BSFC seems to peak with the 65-mph speed.
I had a E-100 Econoline with a carbureted 300 CID six with 4-spd OD,steel radials, which could not be coaxed above 17.6 mpg at 55-mph (Denton,TX to Colorado Springs,CO and back).