That 10 mpg and 50% gain tallies well with the EPA figures for that car at maybe 23 highway.
I think you lucked out and put a simple generator on the right car....not putting what you did down...it's more than I've done which is zero so far in terms of hydrogen.
From what I can gather...there are some "groups" that focus on trying get get the greatest gas flow they can...and they get some very high flow rates but use some complicated tech to do it....and there are some "systems" like the HAFC where the simple H generator only puts out 1 L per minute...the other mpg gains coming from fuel heat/magnetics/sensor tweaking/fuel additives, etc.
Just ran across a video where the guy stated that he found a bunch of pizza cutters with ss disks (Dollar Store?) and bought them for the disks.
I'd stay with your simpler h generator with improvements and tweak in other areas...maybe try an EFIE?
You might try magnetics and fuel additives also. Oil additives...engine/trans/diff. PCV jar. Aero is also a good area to explore.
Might check this post out for some other Volvo ideas?
http://www.mpgresearch.com/viewtopic.php?t=1119
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Leading the perpetually ignorant and uninformed into the light of scientific knowledge. Did I really say that?
a new policy....I intend to ignore the nescient...a waste of time and energy.