Well the first thing is to work out how much energy it requires to keep an average vehicle cruising at(say) 65mph. According to this source it takes around 20HP to cruse at that speed.
Let's convert that to electrical energy by multiplying by 746 (the number of watts in a horsepower).

We get 14,920, or roughly 15KW.
Now, if we want to replace 40% of that power with energy from HHO gas, we'll need to use at least 15KW x 0.4 which comes to 6,000 watts (6KW).

If we assume that the electrolysis cell which converts electricity into HHO gas is 100% efficient (which it certainly isn't) then that means we'll need a massive 6000W/12V or 500 amps of current to make that much gas.
Suddenly those 30A wires are looking rather inadequate aren't they?

What's more, since the average car's alternator can only deliver about 80A of current, this means the battery would have to deliver the other 520A and (in the case of even a good 80AH unit) would be flat in under 10 minutes.
Of course these simple calculations ignore the fact that electrolysis cells are not 100% efficient and the even more important fact that the average internal combustion engine is only around 30% efficient -- so even if we delivered 6KW of HHO gas to the engine it would only produce under 2KW of actual power.
With these inefficiencies taken into account we'd actually need a staggering 1,500A of electrical current to generate the necessary HHO gas to reduce our fuel input by 40%.
So clearly the math doesn't add up. There's just no way you can extract enough electrical energy from your car's automotive system to create the gas volumes needed to create any meaningful amount of energy.