I just got back from a trip to the Florida Keys (over 1,000 miles one way), and filled up at various places, buying mid grade (89 octane) and premium (mostly 93, but one was 92.5). I was a bit disappointed with the mileage, which was hovering around 21. It didn't seem to make any difference between midgrade or premium in general, but I got the best mileage from Shell premium, at just over 23, and this was driving with a combination of 80-85 MPH speeds and slow crawls through the horrible traffic! Someone else had posted better mileage with Shell midgrade, so I'll probably try that at some point, but Shell stations are not on my regular driving routes.