[RedDevil]

American E10 does not have ány water mixed in, right?

I ask this because I had one tank of German E10 (10% pure ethanol) of which I could smell the ethanol quite clearly when I filled up and on a 'cold' start with open windows (yeah, that cold )
I am now on my 3rd tank of hE15, (15% hydrogenated ethanol, so 14.4% ethanol and 0.6% water) and it does not smell that strong at all. The water content binds the ethanol so it is much less volatile.

A bottle of pure ethanol will empty itself in an hour or so if you leave the cap off.
It evaporates so fast that it is hazardous in the presence of open fire.
Therefore it is often sold as a 96% mix with 4% of water. Then it behaves like a liquid instead of a gas. That same mixture makes up the 15% of hE15.

Gasoline does nothing to bind the ethanol, at least not like water does. So the ethanol in E10 is still quite volatile.
I can imagine that if your tank vents even a tiny bit, you'd lose the ethanol much faster when it's pure than when mixed with 4% worth of water.
That could explain why some people get worse FE on ethanol blends. It simply refuses to stay put. All that is left is the gasoline.

In that case mixing E10 with 0.4% of demineralized water might improve your economy. Just add water, boys!
WARNING I accept NO responsibility for this, if you try this on my advice it is on your head.!! I cannot test it as my blend already contains water.

Certainly do not just pour it in the tank, the last thing you'd want is a layer of pure water on the bottom of your tank entering the fuel line at random moments.
Rather take a sample and experiment with mixing that with water and see if it easily mixes or not. Ethanol is very hygroscopic so I think it would, but I can't test it as mine is already hydrogenated.

It sounds too good to be true.
Just add a bit of water to get a slightly better FE...? Really?
But as I explained above, the water plays a vital role in making the gas less volatile. When it burns it will just add to the volume of the exhaust gas, slightly reducing the heat but increase pressure for the same amount of (original) fuel content.
If it does work, that would be something