My numbers are pretty similar to Supercuda and Hotroddave. Mid 20's with ease but from a very well worn and weak stock 318 in my 87 Ram.
Swapped in a 360 Magnum, w bolt ons. 670 Avenger, M1 dual plane and small headers to 20mpg.

As for power, the first stage was just the engine swap. So my numbers was about 5mpg loss but it went from (guessing) a 18 or 19 sec et to a low 15.

Later swapped the rear gear and overdrive manual transmission. Gained 3 mph and now in the high 14's and straight highway is 23 mpg.

With no other changes, bigger engines use more fuel. I don't care about fuel used when accelerating. Most of your average mileage is from driving down the road. During that time smaller is better.