It depends your driving conditions and style. I saw about a 1mpg gain when I went from 20x10s wrapped in E-rated 33x12.5s, to 17x8s with P-rated 285s. I now average about 22mpg driving it, and my wife averages about 19-20mpg. This is very close to what we saw with the stock wheels and tires (which we only had for a bit over a month). The current setup weighs very close to stock, while the 20s and 33x12.5s added about 50lb per corner (and it was definitely noticeable when driving it!).
I guess that translates, for us, to 437-509 miles per tank using all 23 gallons.
But, miles per tank is not a useful indication of fuel efficiency because it is difficult to consistently fill up from the same volume. One person who considers 1/8 of a tank "empty" might only go 250 miles between fillups, while another who habitually drives for 3 days with the fuel light flashing might get 350 miles between fillups. Without being able to define "empty" as a datapoint, these numbers are thus not helpful for any real assessment of efficiency.