I have a 93 F150 I bought new, from day one I checked the mileage at every tank fill up. It is a good indicator when it drops from what is the normal mpg of 15 that something is not right. It could be anything, plugs, wires, injector or a sensor. Last time it was the number 4 fuel injector. Over time if you keep it long enough you can tell if somethings not right, you can fill it in how it drives or sounds. I don't have it with my Jeep yet, only had it a couple of months. I did find a bad bushing and worn gear on the oil pump drive. It was making a ticking sound, not really affecting drive ability yet. Your mpg decrease could be winter gas as others have said but I would check things out to make sure. Over time when the engine hesitates or coughs you will say that sounds like..... The Jeep will grow on you over time and that experience will make it easier to maintain, notice I didn't say easy. I am tracking the mpg on my 06 Wrangler Rubicon and so far after roughly 3K miles it is getting 15.5 on average per tank. Not to bad, some get better and some worse, its just what my Jeep gets and none are alike.
