I've been using a HHO generator with 4 spiral plate cells in series with 8 headlight bulbs. Results are that I get about 1 liter per minute and from 15% to 20% better gas milage. It seems to work better if the unit is off until I have gone 2 or 3 miles and warmed up the car then I switch it on.

My unit is described on my webpage
http://www.maverickexperiments.com/

ted