I have a 2006 2500HD Duramax 6" lift with 34.49" Nitto's. Bone stock with Opti Lube XPD Summer Blend Best highway mileage so far is 20-21MPG with foot out of it. I was thinking of taking lift off, going back with 245/75/16. Less drag with stock suspension, Less friction with smaller tire so...... Any guess on mileage difference by doing so ???