Welcome. You should be able to beat your short term goal pretty easily with basic hypermiling practices.
HHO is not so easy, and the its effectiveness has not been clearly proven. Existing members who already practice other hypermiling techniques/modifications and try HHO (open-mindedly and scientifically) tend not to get results. My mind remains open and I may experiment with it some day, but most of the success stories come without sufficient data and are guerilla advertising, blogspam, or from someone who has not even tried to measure their results.
putting HHO aside, there are many other EASY ways to increase mpg.
one is increasing tire pressure
set your personal speed limit to 55 (if your streets allow)
get into top gear as fast as you can without causing your vehicle to go into open loop
grill block (experiment with this and don't overheat your vehicle)
there are many more. if you are willing to put some money into it, get a scangauge. they are great little devices that will tell you in real time what your MPG is. that was my first modification. it is a little steep though $170 or so but it is well worth the money and it works on any vehicle 1996 and newer.
welcome to the site, start a gas log and do lots of research (that is how I got started)
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