98 Chevy 5.7

First you find a loonnggg hill, then you start at the top......

Actually, I have a 99 5.7 Vortec in a 4x4 Tahoe, and on the highway I get from 17 to 19.6 depending on driving conditions, load, and speed. I tried a free flowing exhaust, and got no help, no change.

Finding the sweet spot, about 2,200 rpm, and being very light on the gas seems to do the best for me, ymmv. Last trip out my mileage varied from

Good luck.

George

Park it and buy a Honda Civic...

There's basically nothing you can do to improve the gas mileage of your truck that won't cost far more than the gas you'll save by making the improvement. You can spend $1000 on a fancy "cold air" intake and free-flowing exhaust, and you MIGHT get 1MPG. It'll take about 6 years at today's gas prices, assuming you use 20 gallons a week, to pay for the improvements.

Unless you want to do it on principle, so the money isn't going to big oil companies, it's not worth it yet.