I know other forum members will blast me, but don't disable AFM on the engine. AFM does actually increase gas mileage. I don't get why people on here dislike AFM, it is almost imperceptable when the engine switches from V8 to V4. I can hear an exhaust note change with my aftermarket Flowmaster muffler, but that and the DIC is the only way I even know the engine has switched. A lot of engineers that are a lot smarter than any of us designed AFM, and it works. On road trips, I routinely got 20-22 MPG in my 2008 Silverado with 20" tires. I only have ~1400 on my 2013 5.3L Silverado, but I am getting ~17 with combo city/hwy driving. I also do notice better MPG with K&N air filters and less restrictive exhaust.