[Nezil]

I've been doing some research in to octane around the world recently, and I'll try and summarise what I've found as briefly as possible:

• US Fuel is rated in AKI, the average of RON and MON
• Most of the rest of the world rates fuel in RON only
• You need to know two of the numbers (AKI, RON and MON) in order to convert between the two, it's not as simple as saying 91 AKI = 95 RON, it doesn't!
• Sensitivity is the difference between RON and MON, and since AKI is an average, two fuels with different sensitivity can have the same AKI
• Although it may seem counter-intuitive, modern engines are said to be operating 'beyond RON'. This means that not only are they more sensitive to RON than MON, but their performance actually improves as MON reduces; in other words, higher sensitivity is important
• The best fuel for our engines therefore will have a high RON, and a high sensitivity (low MON)

As an example, the UK fuel regulations state that regular fuel must have a minimum RON of 95, and a minimum MON of 85; premium fuel must have a minimum RON of 97, and a minimum MON of 86. Given that we have these two numbers, we can calculate both the AKI and the sensitivity. Regular = minimum 90 AKI (sensitivity 10), Premium = minimum 91.5 AKI (sensitivity 11).

As another example, Premium CA 91 fuel has a minimum AKI of 91, and the standards require a minimum sensitivity of 7.5. We don't know the RON and MON, but we can calculate them from the AKI and sensitivity to be 94.75 RON and 87.25 MON.

Obviously these are the minimum specs. A UK fuel could have a high MON, low sensitivity and be very similar to CA 91 in terms of octane, but I suspect not.

I found a very recent document from April 2018 detailing the testimony of Chet Thompson, President and CEO, American Fuel & Petrochemical Manufacturers to the U.S. House Energy and Commerce Subcommittee on the Environment. The document is titled "High Octane Fuels and High Efficiency Vehicles: Challenges and Opportunities". In this article, The AFPM proposes a nationwide move to a 95 RON standard for fuel because it would provide a greater than 3% efficiency gain for modern engines. 95 RON is proposed because it is a good cross over between cost of manufacture and efficiency gain. The whole article, linked here is worth a read, but perhaps most importantly though, was this paragraph:

In evaluating the difference between 95 and 96-RON, it became increasingly clear that California is an important limiting factor for increased octane. Specifically, California’s air quality emission regulations limit the ability to increase the octane rating of the base gasoline to achieve more than a 95-RON E10 gasoline standard, and even reaching 95-RON E10 for all gasoline in the state is a steep challenge.

If reaching 95 RON for CA is a challenge, that effectively means that the fuel here is at the minimum sensitivity limit of 7.5 - 94.75 RON / 87.25 MON. This is significantly inferior to a UK base fuel of 95 RON / 85 MON and way inferior to UK premium 97 RON / 86 MON.