You may be able to pick up a mile per gallon or maybe two. As stated above, there are many factors. My experience with my truck gained about 4 mpg. I went from a tired 5.7 TBI setup to the 6.0 liter. I was getting around 12 mpg on the highway before the swap and now am getting around 16. That's as long as I keep my foot out of the throttle.