We had over 20,000 miles on our 2015 Silverado crew cab 5.3L 4x4 truck when I installed it. I changed out the factory oil at 7700 miles, as per oil change monitor. Switched it to full synthetic, Castol Syntec 5/30, with Dexos, and change it at 4,000-4,200 miles,(ignore the monitor and reset it) and only using an A/C Delco oil filter. We drive long trips, 650 miles one way, back home, many times, so we got a good feel about the fuel mileage with the stock air filter and stock air box, and NOT EVER using cruise control. It will average 17.4 city and 27.8 hwy (@ 80mph) with the ethanol blend fuel (SUCKS) with the best fuel mileage so far, has been 30.7 mpg (+75 mph), running straight gas, no ethanol. This engine in not an E85 engine, and I think the ECM fuel table is set a little better because its not an E85 engine. We have put another 3,000 since installing this air filter, driving short trips on the weekends, and it has increased fuel mileage about 1.6 city and 3.3 miles per gallon hwy. In 2 weeks, we shall see what the change is on a long 785 mile one way trip across Montana.  Well, the best mileage to date with straight gas is 34.7 mpg. Our G.M. dealer took a picture of the MPG mileage monitor, and asked how we managed that kind of mileage. I told him, straight gas, and I don't use cruise control.  2 reasons,  1)  I can see a hill or raise in the roadway, and I can accelerate just a little, were the computor will notice a load, and then accelerate, using fuel, 2)  I like to have my foot on the accelerator pedal, because you don't know what may happen in a split second, and have managed to stay out of trouble doing it this way.