I've been researching this for a while and can't find anything about it. I know that the MPG is considerably higher then alcohol and ethanol mixes. What I'm trying to figure out is if the cars are tested with 100% gas or the mix?

 

Not really the answer you are looking for but...
Obviously most test you find on the forums are done with a mix since that's about all you can find these days not to mention if you find pure gas its really expensive compared to say a 10% ethanol mix. if it was me I would have to make the assumption that all the test I find are done on a mixed gas. I personally only have 2 stations anywhere close to me that sells pure gas and its an 87 octane I run only 93. Now if you want to do your own test you can use http://pure-gas.org/ to try and locate pure gas. Also I am not so sure you would make large gains in mpg on pure gas vs a mix but that's strictly an opinion not to mention what if you run 87 pure vs 93 mixed octane?? I see a ton of variables that could be considered.
Good luck on your quest for an answer.

 

I assumed he meant for EPA Mileage/CAFE testing.

 

More than likely it is done with 90% gas and 10% ethanol as that is what is required by the government.