Oh, BTW, asked a 2wd/king cab frontier owner at work, he gets around 20-21, but he is INSANELY obsessed with fuel mileage, so he drive to match, and he has the uber efficient (compared to our iron lumps) QR25DE.
I average 23 MPG if I keep it on 55 MPH. I have a 1997 single cab 4wd, 5sp, 4 cyl. and drive an average of 100 miles round trip a day to work. This truck has 175000 miles on it and while I change the oil religiously it hasn't been tuned up in about 2 years. This is based on all hwy. miles driven in the mountains of N.C. Once I get up over 60 MPH I find the mileage goes to about 19 MPG. I had a 1983 toyota 4x4 that averaged 28 MPG but that was in the day when these trucks were built more for economy and less for power. I drove like an old lady out of necessity because of the $4.00 per gallon gas prices we experianced this summer. I do it now because I think it became a habit.