I was under the impression that under EPA and USDOT regulations, the summer blends increase in ethanol content and thus your fuel economy goes down a MPG or two.

During the summer, since people drive much more (summer vacation road trips, and the heat tends to foster ground level ozone), the ethanol content goes up to help curve the amount of emissions from straight dino gasolines.

More ethanol = worse MPG, less ethanol = better MPG.

 

You know I thought the same thing but then some guy at work explained it to me and said it was the opposite. He must be right because there was a noticeable instant increase in mpg once the summer blends came in for me. I was at 20.5-21ishmpg until spring and then I went to 22-23 almost right after the summer blends showed up. Honestly I have no clue what the scientific reason is but I know I saw a good 10% boost in summer.