What state are you in? CA 91 already has E10 for all unleaded fuel. Not sure where the 20% number came from.

I’ve done this a couple times when I got a bad tank of gas and had OAR move off -1. No more than a gallon.


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No live in NY have 93 octane here with E8 or e10 mix usually but figured maybe a gallon during hot summer months may help boost octane slightly. Torco or race gas are great but I hear they are bad overtime on spark plugs and catyletic converters. I hear the cars can handle less then 20% ethanol so maybe going from E8/10 to e15 or so may help some

 

search for splash blending. It's been discussed here before in some depth.

 

Thanks , looks like 1 gallon e85 to 91/93 according to that does give a little extra timing

 

Lucky you with 93 octane readily available.
Seems your question has been answered.

Octane Boosters < Ethanol

 

This is what I do - take it down to fumes, put in one gallon of e85, then the rest is 91 (which is e10 for us) and I get E15.