Depends on the engine. The early 70's still had big block options but if it's a stock 350 (L48 or L82) around 18 mpg or so average and a little better highway assuming reasonable acceleration and speed (but that won't happen because it's too much fun to drive otherwise). There are lots of reasons to own a Corvette but fuel economy isn't one of them.
I have a Mitchell interchange book. It lists three different fuel tanks used through those years. Two of them are listed as 24 gallon tanks. Probably all three are. but there are two additional choices for you. The 68-69 vetts had a 20 gallon tank that will fit if used with the mounting straps, and a 36.5 gallon tank also requiring the straps. I would suggest contacting Mid America Corvettes for more exact info when changing from original. Good luck.
corvette stingray
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