Trying to save money on gas by making expensive changes to your vehicle is a waste of time, especially if you have a gas hog to start with. Regearing, buying new tires (retireing?) etc would all take forever to break even, especially with fuel prices the way they are. Do them if they help you with other things too - like better off roading, but don't do it to save money.
BUT, spending the money to gain some extra RANGE, could be worth it to you. In my 80, if I squeeze a couple extra miles per gallon, it gives me an extra 50 miles I can drive without running out. That might be the difference between making it to the gas station or being stranded. Of course the best bang for your buck to extend your range is carrying more fuel. But in general you are right. Smaller, skinnier tires, less aggressive tread, lower weight, lower lift, factory bumpers, no roof rack, will all increase fuel mileage to some degree. The better mileage your truck originally got, the bigger affect the mods could have. Your truck was optimized to get good MPG in stock form but some trucks you can do any off-road mods and they still get the same mileage. It's not really worth the headache to think about mileage too much. Make the mods you think are cool and/or necessary, and if you need to get further, take more fuel.
I find that driving style can affect my mileage more than anything and is almost free - it only costs a little extra time to get where I'm going.
