The first thing I saw when checking out the site was the company's guarantee:
"Did You Know Your Car Engine Wastes 20%
Of The Gas You Buy? - Ethos Is Easy
To Use, And Starts Saving You Up To 70?
Per Gallon on Gas - Guaranteed."
The bold claim is the first statement that would edge me away from giving something like this a shot. The statement is purposefully misleading in "guaranteeing" an improvement of "up to" a figure.
I also tend to stay away from offers that market with a 'testimonial' style rather than something that might provide the scientific articles to back it up.
From my reading, emissions of unburnt hydrocarbons are between 1 and 2%, so the dramatic reductions in fuel usage would be quite unlikely.