Silverado 1500HD Gas Mileage

you can count your blessings that the K&N improved your mileage. The 6.0s are getting much worse mileage than the sticker suggests. Even if you added all of the aftermarket goodies and gimmicks, you might get 1 mpg more, but I wouldn't hold my breath.

Choose gas stations that don't put as much ethanol in their gasoline. That is a killer for fuel consumption. You can usually research that on the 'net, but usually that gasoline is more expensive which offsets any mileage you gain. Inflate your tires to halfway between what the manufacturer recommends on the doorsill and the max pressure on the tire. Choose a narrower tire with less aggressive tread. Or sell it.