Why am I losing so much gas mileage? Bad gas maybe?

I have a 02 Nissan Xterra. I usually average anywhere from 220-250 miles to the tank. About 2 months ago I got all brand new tires, had the oil changed as we as the air filter. The car runs fine...no knocking, it idles just right however over the past month I have noticed that I am getting more like 160-180 miles to the tank. It is not leaking anywhere. I have always gotten my car at QuikTrip. I know the weather here in KC has been cold but to lose so much? I filled up tonight and was already down to 3/4 a tank after 35 miles. Anyone want to take some guesses?