Done right a 318 would get better mileage then that, My 302 with an old worn out carb is getting 16-18 mpg in the city and 24-26mpg on the highway with 3.45 gears in back. With a 6 speed you could probably hit in the Mid 30 mpg with a 318. Before the carb went south I was getting around 26-28 mpg in the city and 44-46 mpg on the highway in Overdrive.