How might I squeeze a little extra mpg out of my old truck?

Engine efficiency. Either through improved intake or exhaust. Basically you're not going to get more power (as that's not what you're after) but you can remove efficiency issues to produce the same power at lower rpm's (which will show a fuel reduction to some very small degree). A flat cover for your truck bed (you don't mention that) that is as rigid as possible (flapping canvas ones cause drag). Worth considering if you don't go offroad - a flat underfloor. Use very thin ali or steel sheet (which can be cheap when very think) and just rivet a sheet underneath the car that doesn't interfere with suspension or steering movement and get rid of all those pockets (like the truck bed on top) that holds air and causes drag. You can do from the firewall back if you want to make sure you don't have to experiment with air out-take from the engine and affect cooling.

My car has an instant and average MPG readout, which I pay attention to, and some things I've noticed that are true of my vehicle - and which might be true of yours or might not be: Getting up to 30, then maintaining 30 for a couple of blocks burns more than getting up a smidge higher than 30 and coasting in neutral for those blocks. Even when I use such a high gear that the engine is practically at idle anyway, taking it out of gear makes a difference. Hills - my car is newer and the fuel injection is electronic. So if the hill is steep enough to coast down in a gear that will raise the RPM a decent amount past idle, the car stops using fuel altogether and runs everything off gravity-powered wheels. Due to another curiosity of my car (engine is oversized), I don't lose all that much extra fuel going up a hill (big engine is going to drink a lot regardless), but I save a lot coming down by having the car in a gear that compromises between low drag and getting the RPM high enough to shut off the fuel, and the result is higher average MPG over hilly trips than on flat trip. Your truck might be similar, or it might not. Experiment with higher gears. I can drive up a hill with the engine idling by putting it in 6th gear and going at 30. I think this is a way of losing money in the long run (straining the drivetrain to save pennies on gas - probably not a money saver, but if higher MPG is a hobby...) But the recurring theme in all of these is that I found them using the instant and average MPG meter, and they are likely peculiar to my car - different speeds and different gears would produce different tricks and sweet spots in different vehicles, so I think my real advice is that yes, looking into installing some kind of MPG meter is probably far and away the best action you can take to improve your MPG. As to how... no idea. Sorry.

5) Throwing comfort out the window. While I'd agree that turning down AC will give some benefit to reduced load on the engine (and hence fuel consumption) going quite so dramatic as this is likely to be a safety issue. Driver fatigue through discomfort is a very worthwhile consideration and if you go as far as to need a glass of iced water before you go to tolerate the interior of your car than it is a distraction and a safety issue. I'd go with windows down at low speeds and minimal AC with windows up at higher speeds. The inverse is true of [not] needing the heater. Not really, no. The car produces heat as a by product and has to reject that heat. It uses no extra fuel at all to create that heat. Rejecting it to the interior of the vehicle rather than directly to atmosphere is only a difference of a tiny current draw to the interior fan. It's negligible. Another thing you can do is ditch your cold air input system. You want your intake air to be as warm and as freely available as possible. This doesn't make any sense. Making your car inefficient will not make it use less fuel it will make it use more for a given driving style. Having cold air in the intake makes more power, but that allows you to accelerate to speed more easily and need less throttle angle to maintain that speed and keep your rpms down. You save more fuel if your car is able to produce the (say) 75 or so bhp required to maintain 50mph at 2000 rpm than if it is struggling to breathe properly and needs 2500 to produce 75bhp. the result is higher average MPG over hilly trips than on flat trip. Something else must be happening there. It doesn't scan from a physics perspective that raising a truck up and down (hence extra energy required) uses less fuel than to push a truck along an equal length flat road. Some other factor (like average speed) MUST be in play here if you see less fuel used going over hills. It's just not possible otherwise. I suspect the flat running is done at a higher average speed and hence aero drag is the presiding factor. I can drive up a hill with the engine idling by putting it in 6th gear and going at 30. This is power versus rpm (allowing for load). If your car is able to produce the power required to climb the hill at a lower rpm then it uses less fuel than it would in the higher gear as long as the engine is still in an efficient load band. If you get to the point of 'straining the drivetrain' then the engine is not running efficiently anyway and you are using more fuel than shifting down.

I'm not trying to intuit anything. It is harder to push a truck up a hill than it is to push it along the ground by a significant factor. Exactly. A significant factor. And if that significant factor is 180% then you can come out ahead. You're already aware that the ICE efficiency is different at different loads. We have a situation where doing the same physics work more optimally falls in the over-powered engine's efficiency sweet-spot by pushing higher load for shorter time.

Weight reduction and ghetto aero are probably where it's at. Look at http://www.motoiq.com/Projects/Volkswagen/Sipster(Rabbit).aspxhttp://www.motoiq.com/Projects/Volkswagen/Sipster(Rabbit).aspx for the kind of stuff you could do. Other possible inspiration: the kind of modifications Miata weight-weenies do. The last possibility I see is doing stuff that will improve the functionality of your truck while also reducing its fuel usage, like lighter wheels, lighter suspension components, lighter brakes. But the added functionality has to be worth it besides the mileage improvement.

Gravitational potential energy being converted to kinetic energy is a far more efficient conversion than the massive losses an ICE generates simply by being in different parts of its power band. Right. Yet your massively inefficient ICE engine produced that gravitational potential energy (inefficiently) by using fuel in the first place (as well as pushing the truck along). So how does that factor? It may be efficient on the way down, but it sure as hell isn't on the way up. And there are losses all the way so it doesn't make sense from a physical point of view at all. I'm not trying to intuit anything. It is harder to push a truck up a hill than it is to push it along the ground by a significant factor. If you are getting better fuel economy in hilly territory than flat land, then you're not allowing for some aspect of the equation. Engine rev band efficiency or ending at an overall lower altitude than you started or some other factor.

This is why it doesn't work. You never recover ALL the energy. Friction and air resistance are significant losses, so it is not zero net energy at all. And yet, it does work. I think you trying to intuit this from habitual rules of thumb rather than thinking it though - you bring up irrelevant things like air resistance, when the air resistance losses will be the same (or higher due to lower altitude) when driving flat; they are not a conversion loss. Gravitational potential energy being converted to kinetic energy is a far more efficient conversion than the massive losses an ICE generates simply by being in different parts of its power band. I haven't personally matched gas-pump output against odometer against MPG meter, but I hang out on forums with enthusiasts of the same model who do, and they report the MPG meter was bang on. People with the same model have also reported better mileage on hills. They didn't expect it either. But it makes sense, once you consider the factors involved.

At 18mpg and 60mph you are burning 3.33 gallons per hour. At idle you will be burning a tiny, tiny fraction of that. Its unlikely your car will use more than 0.2-0.5 gallons per hour purely at idle.

you recover all the energy you initially expend. There are losses in any conversion of energy. This is why it doesn't work. You never recover ALL the energy. Friction and air resistance are significant losses, so it is not zero net energy at all. such that the fuel burned just to keep the whole thing spinning is non-negligible - nearly as much as the fuel required to push the car along a flat I suspect you came to this conclusion because the engine is at idling speed? That doesn't scan at all because the fuel used to maintain idle is not purely linked to engine revolutions. 750rpm at zero throttle uses enormously less fuel than 750rpm with 1/4 or even 1/2 throttle. There is no way that your truck will push its way up that same hill without touching the throttle, therefore much more fuel is required than 'just to keep the engine turning'.