Definately 98 octane. I've tried a combination of 98 and e85 at a ratio of 6 to 1.( 6 parts 98 and 1 part e85). I noticed improved throttle response and at partial throttle the revs build to a higher amount. But I used to race (go karts) and I'm more sensitive to changes than the average rider.( some claim it makes no difference). Never run straight e85 because of issues with corrosion. And you'd need programmable fuel injection or it would be way too lean. Have fun experimenting. You can also try a mixture of different octanes. Ie; - 91,95, 98. 91 gives you more bottom end but no top end. With the reverse being the case for 98. I wish they still made 100 octane. Eg Shell Optimax. It was a great fuel with good bottom end, top end and fuel economy. Ride to live. Regards Wazza.What octane gas should I use for my 2012 CBR250?