I have a 2010 F150 with 5.4 engine. Changed exhaust systen and k&N air filter. My gas mileage is right around 14 mpg for town and trips. Any idea how to get trip miles up around the 18 mpg you guys are talking about. Tnansmission also keeps shifing up and down. Have LT305-55R20 tires on

If you lost the exhaust back pressure you may need to add a mufflar designed to increase back pressure a little bit like a mufflar designed for towing. All auto's but even more so for these newer cars/truck with the computer modules are designed to run better with this back pressure. No need to increase the size of the exhaust size if the engine is not been Moded out. They already come with enough size pipe, Cars back 30+ plus years ago need size upgrades and the bends were less than sufficient. The last 30 years though with the computer systems they are robbing the average hot rodder of fun and economy by making them only to be better fuel consumption and reliable with the systems they put on it and worse without them. At least this is the way I see it, since the late 1980's. I use to put custom exhaust on my cars and have fun when I wanted too, and get good gas mileage when I wanted too by lifting my foot off the gas some, Now days the only way to that is to change your parts back to stock.