So the other day I'm refueling my rental car before returning it. I notice the station is selling E85 (generally, gasoline that's supposed to be 85% ethanol, but sometimes only 70% ethanol) for about 15% less than regular unleaded. I look at the gas cap on my car, and I see it's yellow and E85 certified, so I think "great -- what good luck," and I gas up for less.

Out of curiosity, I went online today to see what kind of gas mileage you get with E85. Basically, it's terrible: one study says you get 28% lower mpg.

http://www.cars.com/go/advice/Story....lAlt&story=e85

So, basically, I stiffed the next renter with a 28% higher fuel bill. Ouch.

Knowing what I know now, is it ethical to refuel with E85? It doesn't really seem so but, of course, how do I know I wasn't stiffed?