The 8.8x9.25 seems to be the fastest, however, most people can't ease off the sticks enough to get the best speed out of it. The 9.25 also is a tricky prop to get a good needle setting for many people. Take some time to practice with it, and a good engine will really haul with this prop. The 8.8x8.75 seems to be the best overall prop for the average racer. Easy to needle, good acceleration, and pretty good top speed.