We actually just rented a 2016 4runner on a trip, pretty similar to mine stock, and I was also curious about the change in mileage. Based on a couple tanks of driving the stock, I'm guessing the mileage has dropped about 2 mpg. But note: I also have a steel front bumper from Demello and a full set of skip plates/rock sliders from Budbuilt -- so there's a lot more steel on here than stock. All of which to say, I'm actually pretty happy that the change in Mpg has been as little as it has.
