[finall]

Read Yettis comments above. If water injection and other stuff worked to improve MPG, all the fleets would be using it. The manufacturers would boost their CAFE (look it up) ratings.

There seems to be some confusion, so here is the skinny......

The only thing water really does is cool the intake mixture, so more air/gas can get into cylinder. It also makes things burn cooler, like someone already mentioned in this thread, so you may need it with some turbos/supers for this reason. Nothing to do with gas mileage. Because of what I said above, it can actually decrease your mileage, but can (in some cases) give you more power since you are burning more gas (not less).