How is hydrogen fuel efficient?

In essence you are correct. The amount of energy returned by burning the hydrogen with oxygen will be equal to the amount of energy required to split the water in the first place.

But that's not usually what they are talking about when car manufacturers talk about the "efficiency" of the fuel. Usually when they refer to efficiency, they are talking about the efficiency of the burning process in the use of the fuel, which in the case of Hydrogen/Oxygen is perfectly efficient.

There is also the benefit of a combustion byproduct which is simple water so it doesn't need any special disposal processes or additional conversion processes.

I agree. But just to add that Hydrogen percentage should only reach less than 4 percent as this can cause massive explosion especially when mix with other elements.

in a car engine, it aspirates air, not pure O2.

BTW, this is not a perpetual motion machine. H2 is required as a fuel to generate the elextricity which runs the electric motor

Well, in that sense, how can this type of car make any sense? The goal of mankind is ultimately to depend on naturally occurring energy that is cheap to harness that will eventually resolve the world's energy problem. However, if I am using more energy to create "clean energy", it's really not clean. This is because factories uses powerful machines to split water, and those energy ultimately came from either petroleum or nuclear which could potential exceed the fusion energy the hydrogen will eventually generate. That doesn't seem to make sense because it's like you are spending more energy to obtain a smaller amount of energy which in theory will never be cost effective.

Well, that's it. You are confusing 'cost effective' with energy efficiency. Commercial power plants can produce electricity very cheaply - especially compared to burning gasoline in an automotive engine.

Hmm, I guess that makes sense. So does that mean drilling and processing fossil fuels into gasoline is more in efficient such that more energy is lost in the process? I still think solar is the best alternative. Just lay the panels and start collecting. There's no need for the continued splitting and fusion.