Faradays does only say something about the amount of substance created by a specific charge, the efficiency calculated by faradays law has nothing do to with voltage.
A month ago i was programming a software which would compute the efficiency of a HHO cell when fed with voltage, current and gas output but i realized that this values are absolutely worthless. the voltage needed for electrolysis is 1.23V + an additional voltage, which varies with material, temperature, current density on the electrodes, water type, ...
1.7V could be the minimum needed voltage, who knows..
The only hard fact one can get is the charge in vs. the gas out.