You're referring to a 4th gen 4runner correct? The 4th gen 4runner and FJ has pretty much the same 4.0l v6 as the what the 5th gen 4runner has, though the earlier version has somewhat lower horsepower ratings.
In stock form, a 4th gen 4runner with a v6 will get about 21- 22 mpg highway and 16-18 mpg city. You start losing mpg's right away as you add tires, armor, lifts, towing, hauling gear, start going up hills, any kind of extra work really. So those numbers I gave are really for best case scenarios; realistically, you'll see slightly lower mpg's with day-to-day driving conditions.