Originally Posted by Bueller
Answer = none. Therefore, lifting is almost guaranteed to have a negative effect on fuel mileage, a fact that is already very well substantiated in the real world.
Sometimes you are arguing with someone who does not live in the real world.
Lay a sheet of paper on the ground and drive over it dead center. If the paper does not get sucked off the ground you have enough lift that it doesn't matter, you are probably in an airplane. The rest of us will notice that the air displaced by the vehicle actually goes that far and will violently move that paper. Aerodynamic forces extend beyond the depth of the paint job.
I can find forum page after page with all makes and models fo trucks where a 20 MPG truck has dropped to 15 MPG after the owner decided to "improve" the truck with lift, tires, lights and generally destroy the millions of dollars invested into tuning the MPG as much as possible withen the constraints of still having a functional truck.
I have seen the flip side, diesel (old 6.9 idi) F250 lowered, shaved, skinny tires, all the aero tweaks, taped seams, filled grill, etc. they were pulling 30+ MPG, nearly double original.