Gas mileage 225 Slant 6

I daily drove a stock 1 bbl slant six for years in a 67 Dart GT, 225, 904 and 7 1/4 with 2.93 open gears. Mixed driving in So Cal traffic and my average was 17 mpg. Then I cut .100 off the head with some work and bigger valves, long tube headers and Holley 1920 rejetted and got 20 mpg.