Seems like fair game to me. The car is designed for E85, so if everybody uses E85 when they rent it, the per-mile cost all comes out similar to regular fuel.

I've never tested regular vs. premium in my own car. Do people get better mileage from premium because of the octane? Are you sure it wasn't because the standard grade was 10% ethanol whereas the premium was full strength? (I thought octane was more about power output, not fuel efficiency. If it isn't obvious from this post so far, I am not a mechanic. )