Don't forget there's more to gas mileage than just the engine.
Example:
Wife's 00 Durango, 5.9 ,46RE , 9.25/3.92 gears.
Wife is a ledfoot, throttle WOT unless she is braking.
She was a consistent 9 to 10 MPG
I could coax 15 to 18 mpg if i tried.
We put new tires on (Goodyear Duratrac"s an...