HHO Generators, Brown's Gas, etc.

Has anyone had success installing one these systems on their van? I cannot get over 14 mpg and am looking for anything that will improve the gas mileage. I have installed a newly rebuilt engine (from GM) and all O2 sensors have been replaced. I also have a K & N air filter. I maintain this van very well. All suggestions will be appreciated. Thanks in advance for your help.