[Edahall]

These trucks were not really designed to drive at a sustained 80 mph cruising speed. With an anemic 350 engine in a lifted 4x4, the engine will be close to maxed out at 80 mph. I would look into installing a larger engine like a Cummins 5.9 turbo diesel or a 8.1 Chevy gas engine. The 8.1 will still get poor fuel at 80 mph but you would have some throttle to spare while the Cummins will still get around 17 mpg at 80 mph. I would also get rid of the lift and get some tires rated for high speeds. Having a blow out at 80 mph in a lifted truck is not fun and sometimes fatal. Also look into upgrading the brakes. Convert to hydroboost and rear disk brakes.