I average ~20mpg in my '03 Gen 3 (3.8l) with no special considerations taken while driving, and with mixed loads and driving conditions. Recently flushed and replaced front and rear diff oil with a semi synthetic blend, and transfer case got full synth. Will be going synth motor oil in the next week or so, and flushing the trans as well. New spark plugs going in when I change the VC gaskets. I've also recently changed the air filter as well.
Getting skinnier taller tires, but they will be a good bit heavier as well, and i just lifted the car 1.5" ...not sure which is worse. Hoping I can squeeze 25mpg out of it just once by refining driving techniques and tweaking tire pressure, etc.
I'm at ~5k-12k ft and normally run a blend of 85 and 91 octane.