If it's the one as your avatar, I would take the visor off to help with highway mileage. Synthetics in the drivetrain can help but it's not going to be earth shattering results unless your current oil is really bad. Skinny tires help, smaller mirrors help, air dam under the front bumper helps. All of that would be after the basics like clean air and fuel filters, good fuel, driving easy.
Honestly though, 17 is not bad at all and better than what most people really get with these trucks when they are honest with their math.
Change out downpipe for aftermarket 3", gut the exhaust backpressure valve, remove the cat, keep the stock muffler and use a 3.5" tailpipe if you are really cheap. The 4" exhaust systems are better but the stock muffler will get you to 400 hp with stage IIs, intercooler and chip. Don't laugh, I actually did it...
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