I see the Civic has a high compression ratio of 13:1 because CNG is 120 octane! Burning the CNG directly in a Civic may be better, except I don't know how "clean" that process is? I imagine it produces NOx, HC, and carbon monoxide just like any other imperfect combustion process. The Civic CNG is rated no cleaner than a Prius C by greenercars.org (and less clean than the original 70mpg Honda Insight).
Here's the CNG-to-H2 production:
(1) Steam-Methane Reforming Reaction
CH4 + H2O (+heat) ---> CO + 3H2
(2) Water-Gas Shift Reaction
CO + H2O ---> CO2 + H2 (+small amount of heat)
(3) In a final process step called "pressure-swing adsorption," carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen.
Steam reforming can also be used to produce hydrogen from other fuels, such as ethanol, propane, or even gasoline.
Personal Lifetime MPG:
~90 the original Insight (retired)
80+ Civic Hybrid (manual)(lean burn)(retired)
70+ Prius G3