the government didn't start recording fuel mileage until the mid-80's so there is no official gas mileage numbers. If you wanted to know for sure I would let the tank go empty (not completely empty of course,) then reset the tripometer and see how long you go on a full tank. Then divide the mileage but how many gallons your tanks holds.
about 12 to 13 mpg(I have an inline 6c).
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