My '62 one ton duallie with a 355,mild cam,Q-jet,4 speed,4.10 gears and 31" diam. tires will do about 11 mpg at 60 mph. Of course,last time I was able to chart it,I was fighting a 40 mph headwind and sideways rain half the way. On a nice day,I could probably get 12-13 mpg.
Speed

1954 GMC 350
1957 GMC 1/2 ton
1962 Chevy C-30
1952 Chevy 6400 dump bed project truck
'98 Harley FLSTC
'66 Pontiac Catalina
'76 Chevy 1 ton Duallie
'84 Bronco II
'78 Dodge W-200
'81 Toyota 4X4 truck