So I googled and read people going from 91 to 89oct and supposedly got a 5mpg upgrade. I'm calling bs lmao. Tried it on twice and it's about the same or slightly worst mpg than 91 and that's having a recent tune up and alignment. I have a 2004 aveo 5speed. Another thing.. I don't know or read how u suppose to calculate your own mpg but the way I done it is I wait till the low fuel indicator pops up. Then I'll put exactly 5gallons or whatever I feel like putting then after it runs out and the indicator pops up again I divide my mileage ran with the amount of gallons. Just saying because I can't see a aveo hitting 40 or even 35 in most cases unless going downhill or people not calculating mpg right. Just taking guesses. I wasn't sure what to expect from my car when I got it about a year ago but it definitely wasn't about 24-27mpg like been getting lol. More like 30 average. But oh well. It is what it is. Wish the rpm were 1000 lower in 5th gear