I bought a 2012 chevy crew cab 2wd with the 5.3l and I'm only getting 15 per gal on highway and I only have 40k miles on it what do I need to do to get better gas milage nothing has been done to it had stock tires but does have a flowmaster on it

 

It will depend on how you drive like no hard accelerations and staying within the speed limit helps. Also the type of terrain you drive on. Hills and mountains will reduce the gas mileage you get big time. I had a 91 4x4 and now have a 04 4x4 and both average about 15 mpg combined city/hwy. Hwy only on mostly flat roads 17ish pushing 5-10 mph over the limit. Make sure your tires are not under inflated. I'm assuming you don't have a camper on it.