Will uneven tread on tires affect gas mileage?

I have a 99' Ford Expedition, and recently my two front tires got hosed by potholes. Fortunately, I had one extra tire and the spare on the back, but my extra has about 50% tread and the spare is new, 100%. Their both on the front left/right for now until I can afford a newer one. Does this affect my mpg having different tread tires like this? (my two back tires are about the same 40-50% tread)