Winter gas, summer gas, tires, armor, all the crap we keep in the back, 3 passengers and the list goes on. 4runners have never been known for their fuel economy... much like Chevy Suburbans. Any change seem to impact its fuel economy. I went from C to E rated tires on my 3rd gen and lost an average of 2~ mpg.
I also live 6 miles from work, ride or drive, my trucks have always been for vacation/trips/fun, if I had a long commute I would have bought a beater 30+ mpg. That said, the last four tanks, I have gotten as low as 13.5 mpg around town and I average around 16, this is by the fill-to-full-click method at the same gas station, using the same pump. I am going back to pure gas, I got better mileage and wasn't running that deteriorative crap through the system.