Ramcharger exhaust 2-1/4 duals or single 3"

I had a 84 ram charger 4x4. 318 2 barrel. Single exhaust. Best mileage was 14 mpg. Put on dual exhaust with a H pipe then it improved to 18 mpg. Put on manual hubs then 19 mpg. I put in a mileage plus torque converter then hit the magic 20 mpg. Kim