Hydrogen Loonies* will want to read this one
NASA on Gasoline/Hydrogen blends and changes in combustion
It goes through the benefits and total energy usage of hydrogen.
TLDR: Gasoline usage goes down, but total energy use goes up in most cases, and only gets low % better under ideal cases (not enough to counter hydrogen manufacture inefficiencies, they were using bottled hydrogen and not manufactured on demand), but combustion & emissions benefits are a real thing.
*I used to be a hydrogen enthusiast/loony. I even made my own very high volume electrolysis hydrogen generators. I kept a proper open mind though (not a 'the man hides this secret tech' open mind, the kind of open mind where you thoroughly explore the merits behind any given argument). This paper ended that particular fantasy, as it was able to confirm why I thought hydrogen should work, in a worst case efficiency scenario (most room for improvement), and all my theories were correct. They also had energy usage numbers that actually stack up, against HHO generation.