To get the beat mileage possible just keep it tuned up, make sure the filters are clean the tires carry the proper pressures and run good quality fuel. Then drive it like there is a rotten egg between your foot and the accelerator pedal. If your wanting more speed then you have to sacrifice something and that usually means fuel consumption, when speeds go up mileage goes down. Now if your made of money, you could replace all the body panels and the bed with carbon fiber units, all the suspension with aluminum alloy replacement parts, smaller and much lighter fuel tank, but to do so would be extremely costly.
You're only going to go faster straight ahead with a more powerful engine and fuel economy will suffer. Before retiring I taught pursuit driving for a large metro PD. We began purchasing pickups and had to teach tactics. Basically, do not pursue in a pickup. We outfitted one ford truck (late 90's) with outriggers. I could roll the thing on either outrigger at just over 30 miles per hour. Just about every student rolled it on their first hair pin turn. Be careful.
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