I don't have enough data and it's no surprise to many that pure gas is more powerful than gasoline diluted with ethanol. I'm just throwing in my useless knowledge. I've been using 87 with up to 10% ethanol in my tank as it's cheaper and there's little choice as it's available at all gas stations. I had recently remembered that there's one gas station that sells ethanol free however the octane is 91 which is not what the dart recommends. I've vowed to not make any special stops and if I pass the station I'd stop to fill up on 91 but use 87 for when I need to make a special stop. I've gone through a few cycles of fueling and had filled up with 91 yesterday when it was on empty & had reset the fuel economy. Before I was getting a reading high of 28 mpg mixed driving and a low of 25 from aggressive city driving with an average of 26-27. I do my best to hypermile within reason as the situation allows and never impede traffic. I haven't kept track of the mpg manually but recalled get 32 mpg on one particular day. Adding 91 to 87 didn't seem to make any noticeable difference. After filling up on 91 and resetting I noticed a high of 32 and am curious what the average will be. Calculating the one trip by hand however netted me at 27 mpg which is lower than what the computer says. I am a firm believer of using pure gas despite the gas difference. It might not be cost efficient. I am using all season tires for rain/snow, spark plugs & engine coolant are due for replacement in about 6k miles but everything else is in order. My car is a 2016 Dart Limited 2.4l 94k miles I keep my vehicle clutter free but I deliver groceries which lowers fuel efficiency.