Ok, I have been putting premium (91) gas for the last 9 months and my last fill up I used the 87 octane and my mpg dropped to 10.2
All city driving, same way to work, School and home. I was getting around 12.5 to 13.5 on the 91 octane. I'm using Chevron and I think the 87 burned faster than the 91....well that's my theory.