I get about 18mpg normally and 15mpg towing. That might be low 1mpg as my odometer is off due to larger tires. The best the truck ever got was 22mpg when it was new with small tires and no lift.
I recently added a Super Charger and mileage is unchanged if I drive with the same temperment. I will be towing for the first time next week. I'm hoping for no engine ping under that load.
I also added a second transmission cooler if anyone is interested. It's an easy install. Engine is a 3.4l as noted in the picture. 180hp stock, roughly 240hp with the SC.
http://www.larsdennert.com/4runner/supercharger/