Modern winter tires do not cause much loss in mileage. The treads are inverse to the rolling surface. All terrain tires are not as efficient that's for sure, as tread protrudes above the average rolling surface.
My winters are a taller tire, and although they may require more time to speed up they do less revolutions at any given speed than my low profile summer wheels. Less RPM = less gas used (assume level ground).
Performance one step at a time.