Just put on 272 miles with E66:
1.0g E10 = .1g E + .9g regular
15.0g E70 = 10.5gE + 4.5g regular;
=10.6g E + 5.4g regular; 16.0g total;
=E66
I know I started with an empty tank because I literally ran out of gas on the way to the station. I only put 1g reg in because I didn't like their prices, then 15g E"85" @ $2.90/g, saving over $11 vs filling up the day before.
I presume it was actually E70 due to the time of year, but at 70+ degrees out it seems summer has arrived.
No issues, not even a CEL flicker. At end of 272 miles, gauge is sitting just over 1/2; call that 8g burned = 34mpg. Winds were light and variable, not necessarily favorable, cruised between 53-58mph.
34 mpg would be a good number for this car on regular. Perhaps the accessory drive mods helped a bit...? Perhaps E85 isn't the road to ruin?