You can find Nasa's test from hydrogen and fuel mixtures here but that link was provided from comments left on this popular mechanics article which has a pretty skeptical author. I skipped through the pdf file but saw mostly talk of emissions and effect on the way fuel was burned, not actual mpg figures. People who commented on the article in the second link said that nasa used 100 times the hydrogen then HHO can provide and still didn't get magajor MPG increases. I’m also really skeptical.. and put my stock into known, proven technologies: Little motor with just-e-nuff hp wrapped in a scion Xa shell.
Funny thing though, that report was writen in 1977 Nasa and included "in view of decreasing petroleum resources, a system using 100-percent reformed methanol might be an interesting supplementary
or alternative fuel system"