It seems like in the interest of emissions, the newer vehicles just do not get MPG like they should. Burning more fuel to produce less pollution, or so it would seem. I've rarely seen a Gen III Explorer get over 20 MPG...Usually it's more around the 17 MPG mark. The Gen II with the same 4.0 SOHC engine and even a little lower overdrive and/or final drive ratio will get better MPG. I had a Gen II for a demo for a while, a '99 Limited 4.0 and that thing would get no less than 16 MPG no matter how hard I flogged it. Out on the highway, I was seeing 22+ on the trip computer (which was located in the center console...How odd) keeping it at 60 MPH. What changed??!? The engine's output is similar...The Gen III got 3 HP more than the Gen II, and I can't see there being that much difference in weight or aerodynamics. Even one of our techs at work has a Gen I with the 4.0 OHV engine, and he gets decent MPG with it. Not as much power, but decent MPG. On a trip he averaged 18 MPG with 4 people and some stuff in the back.
You'd really think that technology would have us going forward instead of backwards in the fuel economy department, wouldn't you?
In all honesty, if my Explorer wasn't a FFV, I doubt I'd own it. I had a '01 Durango 4.7L previously, and it got the same if not slightly better MPG than my Explorer does on gasoline, had more room and weighed about 800# more. But, the Durango and Dakota also got "dumbed down" starting in late 2001-early 2002. Ask a '02+ Durango owner about their MPG, and get ready for some cussing. One friend of mine had an '03 and averaged 10 MPG, getting a personal best of 14 MPG on a trip. But...Her engine didn't ping under load on 87 octane! Mine did...So, I just ran 89 in it, and got half again as much fuel economy. They re-calibrated the PCMs on the later models (and offered it as a fix for pinging on the early ones...We called it the "death flash") to run richer with less advance, and the end product was horrible mileage and a bit less power.
The carmakers really need to get their heads out of their arses (and Big Oil out of their back pockets) and get some efficiency back in these engines. It can be done...Heck, it was being done before the emissions laws got tighter. There has to be a way to make an engine clean and efficient.