I googled EFIE and it came up with
this as its first result.
No insult intended to George at all, as he is as yet an unknown quantity and therefore deserving of at least common respect, but that lands EFIE squarely in the Snake Oil store at the Mall of Broken Promises.
If I'm reading it right, the EFIE device could be entirely responsible for MPG gains by fooling the engine computer into leaning out the engine. I could do that with a carburetor and a screwdriver in five minutes, or a WAI on another car in an hour or so. HHO would not enter into the equation; in fact the EFIE is touted as "correcting" problems that running HHO would otherwise cause.
Here's where I am on that: Engine + HHO = bad mileage, + EFIE = back to normal. Couldn't we have skipped some steps and stayed at normal?