There are numerous things that cause MPG's to drop, tires- the type, air pressure, unbalanced and just being bigger. The next one could be the air filter type, if it's dirty or it might be a cheap one. The gas that you use, I always try and stick with the same gas station and i never let my gas tank get below a 1/4 tank. Fuel injectors could be dirty, but not clogged there's a big difference. Cold weather??? I don't know.
I get about 17 on the highway and about 15 in the city, but then again I am carry extra weight because the winter time, my emergency box which weighs about 80 lbs, my hi-lift jack, air compressor and other stuff. I don't worry about gas mileage unless it gets below 14.
Just my .02