contrary to popular belief, gas mileage is more closely correlated to total vehicle weight and aerodynamics.
While accelerating from a stop the only work being done is overcoming mechanical drag and accelerating mass. (F=ma)
More mass=more gas.
When you're on the highway (flat highway) the only work your
doing is overcoming mechanical drag and wind resistance.
Gigantic SUV frontal surface area=more gas.

If you stuck a suburban engine in a civic it could get 30mpg.