On my 2000 Wrangler (4.0, auto, mainly a road car)... normal highway mileage with the hardtop on is 19+ (never quite broke 20).
A few weeks back, it started cranking a bit longer when starting. Not much, it just didn't fire on the first couple of turns. I figured "dirty injector, or some water in the gas" and poured in a can of cleaner.
Then I took the Jeep on a 300-mile round trip to pick up a camper. On the way out, without the trailer, the mileage was 23.4 and I figured "tailwind". On the way back, I thought the Jeep was a little sluggish, but that was because I was towing 1000+ pounds. Mileage on the return was 17, what I generally get around town. OK, happy time.
Next morning the thing barely started at all, so I brought it to my mechanic (local independent guy). OB reader said low fuel pressure, lean mixture, and Cyl.#2 missing fire. Fuel pressure at idle was 29 psi, book calls for 49. Long and short of it, I had a dying fuel pump.
SO... the moral of the story is, if you notice a sudden increase in fuel mileage, get ready to replace the fuel pump (and the filter, and the regulator, since they're all in one unit inside the tank).