I have been in the same position as you except my way out was different. The 2F as mentioned above will not give a mileage better than 6km/liter, that too will a toned down driving style.
Running on a 3B currently has proved to be a better option. Current mileage is around 10km/liter with 31x10.5x15s.
IF fuel consumption is a big deal, consider an engine swap, your 2F will only let up to a certain extent.
less weight that lambcrusher, of course, probably less drag, too, in a '72 softtop FJ40 - 2F, 4speed, 4.11 gears, 33s - best ever was ~18mpg on the highway (including Colorado) - typical is more around 15mpg
too lazy to calculate liters per 100 kilometers (or anything else)
60 miles over and through the mountains to the desert on two lane roads at 45 - 55mph. 30 miles of mostly 4X4H (a little low range) crawling around in washes and canyons. 60 miles home again. 32 inch tires (285-75-16) 4.10 rear end 2F 78 mostly stock FJ40, Aisin carb with all of the California smog stuff. What a great day! 11mpg on the nose. I was kind of tickled to get better than 10mpg.