Well going by this thread I guess I got pretty darn good mileage on my long cross country trip a few months ago.
Considering I was hauling I figure at least 800# in the back plus my wife and I and a 12# Chihuahua I still managed to get an average of 19.83mpg over 7770 miles and 391.66 gallons of gas burned.
I filled up exactly the same each time, fast as the pump would go until I got to where I felt I was within a few gallons of full, then cut it back to about 1/2 until I got the first click, then about 1/2 or less to the 2nd click and that was my mark. I think you get a more accurate measure when you slow down the fill rate a bit as some gas stations the gas is flying out of the pump and it seems to click really early.
I did manage on ONE tank to get 21.47mpg from Bend towards Yellowstone which was very flat and slightly downhill in spots. The 800# might have even helped my mileage on this tank just from momentum.
I think 2-3 other tanks I got just over 20mpg with the majority being between 18-19mpg.
I used 2 bottles of the Amsoil PI fuel additive, one just before I left and one after about 4000 miles, not that I think that made any real difference I just put it in there to keep injectors clean.
Though I never found out for certain what my "check engine" light was about I came to the conclusion that it was most likely caused by a dirty brake sensor that I probably got while in Oklahoma driving though mud holes.
My best guess anyway as it has never come back.
http://www.toyota-4runner.org/3rd-gen-t4rs/143963-back-7-770-mile-trip.html
As far as I can tell I am still only getting about 13mpg 100% city in Raleigh NC, even though I am getting slightly better Highway mileage than I used to before changing the intake and the muffler, it appears as though that only made a real difference in just highway mileage.
Maybe because of constantly getting more freer flow of air IN and OUT at highway speeds?
The OP made my brain hurt.