Ummm, some basic physics comes into play here. Gas mileage is related to how much work is being done and at what efficiency. A 2 cylinder engine operating at 30% efficiency moving a 3000 pound total load from A to B up and down hills, overcoming friction, etc, will get EXACTLY the same gas mileage as a 4 cylinder engine operating at 30% efficiency. A true 2 cylinder might save on a bit of weight and internal friction but that is all. What you want is for your engine to operate at peak efficiency. This is hard to do given the wide operating range of a typical engine. Unless you engineer the alternation properly, you will likely reduce efficiency, not increase it. It has been done by GM but it adds complexity, costs more and really doesn't help much even when properly engineered. A hybrid electric does better because the engine can operate more often at peak efficiency revs.

Consider also the TOTAL amount of gas your car will deplete from birth to death, including mining and processing of raw materials, fabrication, and so on. You want to minimize that figure, maybe, if you are concerned about our limited oil supply. Altering your engine to run on 2 cylinders will more likely just shorten its life, pollute the air (because you would be running the car far off optimal conditions - try passing an emmissions test on 2 cylinders!), and deprive you of sufficient power when you really need it (to get out of harms way, for instance).

You would be better off spending the time and money keeping your car maintained, keeping the tire pressures up, deciding to take a local vacation rather than a long drive, etc.

Just my $.02.

Bill