RON=Research Octane Number. Basically, a calculated value. Generally a higher number
MOT=Motor OcTane. Variable compression head motor that is indexed to show real octane performance with an internal combustion engine. Generally a lower number.
AKI= Anti-Knock Index. RON*MOT/2 (average) Method used in the U.S.
91 AKI is generally 95 RON, but as it depends on the MOT value, it could change.
Nice way to advertise octane numbers, but keeps everyone in the dark as to what's really happening.
If you hear knock, go to the next higher grade.