mpg single vs super six

when i switched from a holley 1920 1bbl to a carter bbd 2bbl, i noticed no difference in fuel mileage. both were/are well tuned and run great. i did see an improvement in fuel mileage when i switched plugs to the ngk zfr5n.


How much improvement with mileage did you notice with those plugs? I might have to look for some of those. Thanks!