You're right, it doesn't make sense that an engine that runs on gas and E-85 would run on either efficiently, even I understand that. The key word I think would be......efficiently.
My wife's brother has a full size scrub flex fuel truck. He is divorced, kids grown and is known for being the "thriftiest" person you will ever meet (read that as he is one of the cheapest B_______s to walk the face of the earth). He drives to Florida from Chicago a couple of times a year and from Chicago to northern Wisc. on a pretty regular basis. He puts some miles on this truck and has determined E-85 must be 20% cheaper then regular for it to benefit him to run E-85. If regular is $4 a gallon E-85 must be $3.20 or less to make it worth buying.
He does acknowledge that anything close to a 50-50 mix in the tank makes it run very poor. It has to have a majority of one or the other type fuel to run half way decent. Cheap as he is I know he's run the numbers dozens of times.
For what it's worth.
Missouri Mike
I'd rather be lucky than good.................
but good ain't bad!!