Edelbrock 500 vs. 600 cfm fuel milage

I am hoping to get about 15 mpg out of it. I'm not that far from it now. Its gotten the best with the 1406 on it. I even thought of an AVS2 . I'll keep playing with it.
Minor things all ad up. I had a 1983 4x4 Ramcharger. Air up the tires, drive like there is an egg under the accelerator pedal. Accelerate away from lights and stop signs easy. Let off the gas when the light up ahead turns yellow. Fresh oil change with efficient oil. Fresh rear diff lube change.