Got it on 83 "Eco" now I filled up with 93, so when I change to "93 performance", do I need to change back to stock first?

 

no. new tune will overwrite the old.

 

Ethanol has a higher octane rating than gasoline. 113 is it's octane rating in pure form, though it does not have the same energy content, so it is less fuel efficient ,meaning more trips to the pump.

If you can find gas without ethanol, you'll get more MPG's, but if the pump says 93 octane, it's 93 octane regardless of how they mixed it.

If it weren't for the fuel pumps and fuel lines in our trucks, you could run E85, but the upgrade would take a long, long, time to be recovered (if ever) in savings due to the price of the parts to upgrade and the diminished MPG's. The stations that carry it have drastically reduced in number as well, might cause you to burn extra fuel just to get to one. Around here ethanol is only $0.30 cheaper than regular gas, but it needs to be about 2/3's the price of gas to save money. There are no stations within 8 miles of me, and none between my home and work. That's a waste of travel and money. People buying ethanol gas around here either only buy it for the "right now" feeling of saving money, or they are the "feel good" environmental types.

You might be able to advance the timing significantly though with E85, as it's octane rating is around 105. That'd be a nice performance boost, but you'll pay at the pump. might be good for the track though...