My personal experience is that the HiHy has more horsepower when I use 89 octane instead of 87, most noticeable between the speeds of 20-50 mph. The engine noise from the ICE is louder and there's less "punch" with 87. I believe the fuel savings comes indirectly. To get the same level of acceleration with 87 as 89, you have to drive the ICE harder, consuming more gas to achieve the same cruising speed in roughly the same amount of time... I haven't tried 91 octain because I'm satisfied with the power I'm currently getting with the 89 octane.