That would be H20 then... Aka water...

Sure you can put a bit of water vapour into your engine, and it will
actually act as a catalyst and make it run a bit better... Better
still, spray it over your intercooler (if you have one) and that will
increase the density of the air going in, and aid combustion.

However, if you want to see the kind of %age claimed on the website,
the best thing you can do is learn to take you foot off the gas a bit,
and to avoid hard acceleration. Throw all your heavy rock track CDs
out of the window, and listen to something mellow instead... And above
all, don't travel in rush hour, leave it til 3am, and then you can
cruise all night!

d0d6y