E85 is a wet-dream of the environmentalists and agribusiness.
Yeah, E85 can be up to 105 octane but, because it's 85% alcohol, you deal with alcohol's shortcomings as a motorfuel: 1) much lower AFR means you burn a lot more of it, 2) lower energy value, ie: less BTU's per gallon.
For E85 to be a "good deal" for the consumer it needs to be priced at 25% or more below the prevailing prices of gasoline and you still don't save any money until you recover the cost of the conversion.
If octane is what you need, you're much better off using racing gasoline. It would be far less complex and much less costly then converting an old car to run on E85.
Worse–it's a net energy-loser. Ethanol contains about 76,000 BTUs of energy per gallon, but producing it from corn takes about 98,000 BTUs. A gallon of gas has about 116,000 BTUs, but making that gallon of gas requires only around 22,000 BTUs.
It's amazing how some environmentalists and politicians have bought into the E85 fairy tail. Well...maybe it's not.