My '75 Ventura custom hatchback got 25 mpg all day long @ 55-60.
That was with the Olds 260 and 3-speed stick; 2.73 gears.
I suppose a 250 I-6 (or 231 V-6 after 1976) with manual trans would do a slight bit better with the same gears, but factory ordered might require a shorter rear gear ratio.
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