318 fuel mileage

Got about 19mpg on otherwise stock 72 318/904 duster (exept exhaust headers). after rear gear swap from 2.7 to 3.2 mpg dropped to 17 to 18 on highways. now I am doing hei conversion and getting 4barrel carb (summit 600cfm+ edel perf intake). I don't know how mpg will change yet, have to wait summer.