Max MPG- slant vs 318/340

Bear in mind it takes a given amount of fuel to move a given weight/shape car down the road at a given speed.
I removed the 258 6cyl/5spd from my Wrangler years ago and replaced it with a tune port 305/700R4. it got the EXACT same mileage before and after (16/18 MPG) but sure was a lot faster after!!