There is a super long thread on here already where everyone posted their mpg, it has gone on for years, ever since '06 or early '07, so let's not start a whole new one of those now (unfortunately, lots of those people had posted to that thread simply a number with not enough information about their mods, what speeds they drive, what altitude they live at, hills, mountains and so on).
From studying all 40 or 70 pages of it, a few things do seem to be pretty consistent:
As Cliffy pointed out, a S/C drops about 2 mpg.
A roof top carrier (not just roof rack, but with a box on top) drops it about 1~1.5. A 3" lift and big tires drops mpg at high speed, and a few hundred pounds of steel front and rear bumpers, and permanently mounted storage, air pump, fridge and so on, inside, tends to drop it 1~2mpg or so, depending on how much stop and go driving one does (dragging all that dead weight back up to speed each time) and how fast one commutes (big lifts, big tires and big bumpers and roof top carriers, have to push more air which resists by a square of the velocity, so mpg between a speed of 60mph is quite a lot higher than at 80mph, over a 12 hour trip.
Range is directly proportional to mpg. If you need more range, keep it stock and keep the speed down. But both are inversely proportional to joy.
FJs don't get good mileage, compounded by the small fuel tank. Too many threads on this familiar, heavily trodden theme.
As Woody Allen's female lead said in What's Up Tiger Lilly?, "I'd call you a sadistic, necrophilic beast, but that's beating a dead horse."