In stop start driving round town I get about 20 to the gallon, 28 on a run on 33" tires, full roof rack and winch bumper. I am in the UK with a manual diesel though;p mind you it costs £1.41 a litre here. ($10.45 a gallon)
You also presumably use a larger 'imperial' gallon.
My bone stock LX450 (1997) got 15mpg (US gallons) on the highway drive from Albuquerque to Las Cruces when I first bought it. My next tank made up entirely of 3 mile commutes around town netted me 14mpg. Most of this is at 4000-5000 feet. I don't know how much elevation impacts economy.
Keep in mind that larger tires also mean your odometer is off, so if you use that to calculate mpg it will make it seem worse.