I own a 2000 Silverado 1500 with 5.3 V-8, keep tires inflated around 40 and use synthetic Mobil 1. If the vehicle is well maintained and you drive conservatively, you should be able to get around 19-20 MPG at sea level with proper/higher air pressure and synthetic oil which reduces engine drag. On long trips on relatively flat roads at or near sea level, I've been able to get 22 MPG max, but overall, I get between
19-20. The smaller engine has about the same gas mileage as the larger engine in fact, the larger engine may do better in hilly terrain or under heavier load conditions because of it's increased torque and ability to stay in OverDrive. Chevy calls for 35 psi in the tires, but I've own Chevy truck for years and they perform better at higher tire pressures. Currently I have Bridgestone Dueler Alenzas, a high end SUV P type tire that can take a maximum of 45 psi. I run them around
40 and the ride is absolutely beautiful....smooth, silent, and excellent handling. They are rated as No. 1 for on road use and will do decently for off road. For serious off road use and heavy load condition, you would be best off with an LT series load range C which can take up to 50 psi. I've use Goodyear GSAs in the past and would run them around 45 psi with good result. Michelin tires have the lowest rolling resistance of all tires and Goodyears, especially the new Forteras with Kevlar have the highest rolling resistance. So for best mileage, choose a Michelin LTX. Their P series can only take 35 psi and that's simply not adequate for a truck . Bridgestones are intermediate in their rolling resistance but have the finest ride. The Goodyear Forteras with Kevlar are even smoother riding but users have reported quality issues with them. I've never own a Goodyear tire that did not have problems with being somewhat out of round. For smooth ride, highway driving, and excellent handling the Bridgestone Alenzas are the best choice. For off-road use, heavy loads and best fuel mileage, the Michelin LTX is the best choice, but they are pricey. Every Fall and Spring Bridgestone has a $75 rebate on Alenzas...I put on (4) P265/16/75s for $600 and that included unlimited rotating and balancing. A deal that cannot be beat. Michelins will run around $800 but the ride with not be as good or the quietness of the tire.