I attribute part of my huge increase in MPG when I did the Magnum/518 OD swap to my CrewCab to the lowering
I did to it while I had it apart which was almost 3" (2 coils cut) & just over 4 1/2" (spring hangers moved up on the frame)

The wind doesn't affect it as much as before for sure, even after adding 40 MPH to its top speed