The [link|http://www.straightdope.com/classics/a3_150.html|Pogue carburetor] and water injectors and the like aren't magic. The Pogue carburator was a fraud.

It's easy to get 30+ MPG out of a 330 HP 428 CID Mustang Mach I. It's easy to get 40+ MPG out of a Ford Expedition. You simply have to restrict the fuel available to the engine and you have to accelerate very slowly. E.g. in the Mach I the owner wired the secondaries of the 4 barrel carburator shut and soldered up the jets in the primary circuit then drilled it out with a smaller orifice. It ran like a turtle, but got much better mileage. It also blew a head gasket shortly thereafter (probably due to detonation).

The best air/fuel ratio is set by the chemistry of gasoline combustion. It has nothing to do with the compression ratio. Emissions are lowest and efficiency is highest at [link|http://www.forparts.com/emission.htm|14.7 to 1] (by mass). Power in a gasoline engine is limited by the amount of oxygen in the cylinders, so for more power lower ratios (e.g. 12.5 to 1) are used to make sure all of the oxygen is used.

Water injectors work in engines that suffer from premature buring of the fuel (pinging, detonation, etc.). It cools the input charge and makes the fuel more likely to be ignited at the proper time by the spark plug. It doesn't increase the compression (increasing compression make premature buring more likely). Water injection isn't terribly effective in engines that don't suffer from pinging. And water injection introduces various corrosion issues, possibly increased emissions, worries about oil dillution and increased wear, etc., etc.

TANSTAAFL.

My $0.02.

Cheers,
Scott.