Reference: US Octane vs The World

I’ve heard a few times that JDM engines run on the equivalent of 100 US octane. I thought that sounded a little weird so I finally looked it up. Turns out, the information is all over the place. However, it seems as though people were mixing their RONs with their AKIs. As with the metric system, Europe and Japan use RON and the United States uses AKI. And AKI is average of RON and MON.

US 93 is equal to 98 RON.

I was a little skeptical of the claimed 100+ AKI octane rating for E85 as well. Pure ethanol is rated at 100 AKI (actually, just under at 99 AKI or 109 RON) and it would need a gasoline mix with an AKI of at least 100 to keep the octane rating from dropping. Again, the information is all over the place for E85. It appears that people are again using RON in place of AKI. My best guess is that E85 is probably in the 97 AKI range.

Math: 85% of 99 AKI is 84; 15% of 87 AKI is 13, and 84+13 gives us 97. Race E85 probably has a higher octane rating, which complicates matters.

US (AKI)	Euro/Japan (RON)	MON	Name
E85 (97)	103	91	E85
93	98	88	Premium
91	95	87	Premium
89	94	84	Midgrade
87	91	82	Regular

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Sources

https://en.wikipedia.org/wiki/Octane_rating#Anti-Knock_Index_(AKI)or(R+M)/2

https://www.goapr.com/support/fuel_guide/

http://www.pencilgeek.org/2009/05/octane-rating-conversions.html

Click to access e85_driver_training.pdf

https://www.researchgate.net/figure/RON-MON-AKI-and-OS-of-fuels-used-in-this-study_tbl1_318813527

https://www.sciencedirect.com/topics/engineering/research-octane-number

Click to access b13_woebkenberg_2-b.pdf