bigger pipe has always equalled better gas milage in the stuff ive played with.i had a slant six valiant four door with a custom 2.5 inch pipe going into a flowmaster knockoff and ending infront of the rear end.it got way better gas milage after i put that on.sold the car to a buddy and he ran more 2.5 inch pipe to the back bumper and said the loss of gas milage was noticeable.