You can run e85 on your car. You will need to tune for it usually you have to add 20-30% to the fuel maps. So you get less gas mileage.

Do some research on stoichiometric.
For gas its 14.7 to one
For e85 its 9.7 to one

Lambda are both 1.0 so if you run e85 without a tune the ecu would probably add fuel to compensate for the lambda value. But you get a code if there is more than 30% correction ether way. This puts the ecu into limp home mode you get shitty ignition timing with this and it all raps up to bad gas mileage.

If you really want to get crazy mpg you can tune gas lean or e85 lean. The only thing is you will not pass emissions for NOX due to high cylinder temps.

As far a octane rating the higher the number the more stable the fuel. Has very little to do with mpg. With 105 octane you can be more aggressive with ignition timing and boost.

You will get the same mpg for 89-91 ruffly because the o2 sensor holds the air fuel ratio around 14.7 at cruise to help with emissions.