Looking for better fuel milage

.............Put on a dual snorkle air cleaner, my mileage went from 17.5 to 21[canadian gallon] with a warmed over in a 68 dart 340.....what converter do u have, maybe its not very effecient.......kim.......

I am running a 2800-3200 convertor...Comp XE284 cam called for the convertor. My air cleaner is open 360 Degrees