When I first got the car 14 months ago, I used regular gas. I was very disappointed at the low 20's mpg I was getting. Then I read, think it was here in one forum or another, that some people were getting better mileage with higher grades.
I drive most of the time with the display showing current mpg. My driving habits have changed drastically. Used to be I only did the speed limit if a cop was behind me or when I was passing the speed limit by on my way to something just short of unsafe, charging up hills, that kind of thing. After watching the effect of such behavior on the actual mpg, dropping down to the single digits, that's a thing of the past. My kids say I now drive like an old lady. Whenever my 16yo daughter wants to use my car, she does so knowing she has to protect my mpg.