I get decent fuel mileage with my Spartan
tuner. I have seen as much as 19.7 highway hand calculated with the latest 275 tune. That was on 35" Toyo Open Country AT's. With my new Toyo Proxes (32"), I made a 200 mile drive and burned less than half a tank. That is also with the latest 275 tune. I ran between 1900-2200 RPM the whole trip.