Class 1-summer time- 80-85% 100-105 octane
Class 2-spring/fall----- 75-80% 96-101 octane
Class 3-winter----------70-75% 91-96 0ctane
And unless you buy certified "Rockett Racing stuff" by the 5 gallon pail or drum the rest of the mix is 85 octane base stock gasoline.
There are published guidlines for when each grade should be sold at the pump in various states-to my surprise in Michigan they are only supposed to fill the tanks with summer grade from mid june to mid Aug and if they don't sell alot you could very well be buying the spring or even winter stuff so IF you were going to build a motor around it You would need to be very careful on what you put in your tank- imagine building a 12-1 motor so you could take adavantage of all the octane and then you get the low grade crap and it rattles your motor apart!!
I am in the middle of dynoing my current project-a 600HP 455 Pontiac with Mefi EFI, I went and tested the local pump E-85 a few days ago and even though they were SUPPOSED to already be selling the winter stuff here, it tested at 83% Ethanol so I bought 52 gallons to use to dyno with as I am doing a E-85 tune for this as a alternative, might not ever use it but I just want to see and this motor is 11.4-1, Smitty