My thoughts are that an all terrain tire ususally weighs more and "probably" has more rolling resistance.Two things that could negatively affect gas mileage.A wider tire may do the same.

Consider a narrower tire that is the same height or maybe shorter.Your lack of gear with a taller tire may actually hurt you as you are giving more throttle to move the vehicle.

Driving like an egg is under the gas pedal is the best way to improve gas mileage.