[LEEVON]

I've had a load of 5.3's, tahoes, 1/2 tons, 3/4 tons, 2wd, 4wd and the mileage has always been from 12-16mpg. Maybe it's my driving, but 20mpg I've never been able to attain.

To your question, the best bang for the buck is to get a custom tune from somebody like wait4me performance, blackbear, nelson, wheatley, etc. This works even better with supporting mods like long tube headers, intake, etc. Also running synthetic fluids in the entire drivetrain can result in slight gains. Before doing any of this though make sure you have a good tune-up. Fuel filter, plugs, air cleaner, cleaned MAF, check resistance on the coil packs and plug wires, tire pressure and whatever else you can.

Don't waste your money on gimmicks like tornadoes, magnets and crap.