18 mpg for a bolt with original equipment is obtainable on the freeway if you want to keep it at a slow 40 mph. If you feel like driving with traffic, 12 mpg is more realistic. I get about 18mpg at 65 mph. But I wasn't able to achieve that until I added an O.D., radials, HEI, and dual exhaust.
They did sell an "economy version" back in the early 60s at the expense of low end power. That included a smaller venturi carb and a 3:08 rear.