I went from the stock grabbers to the BFG AT's and I still get about 250-260 miles per tank. I will up at 15 or 16 gallons so that equals about 16 mpg. I pretty much always have gotten this mileage since day one with the Xterra. I try to drive conservatively at all times. Thats odd that you would experience such a large drop in your mileage though. Its a mystery if you ask me.