Why Is US Fuel Lower Octane?

Despite living in New Zealand, I’ve been on holiday to the United States several times (and have every intention of going back again now that Covid restrictions have lifted). As someone from outside of the United States, there are many different things about America that I find interesting – sometimes in the more mundane aspects of life, such as wondering why American toilets have so much more water in them, or getting my head around tipping etiquette.

One thing that’s always interested me is why American/US fuel seems to have such a low octane rating, at least compared to what I’m used to at home. It’s a tragic thing to even notice I guess, but as a car enthusiast this has always bugged me.

I recall a number of years ago doing a road trip holiday with an American friend, and filling up their turbocharged Subaru with what I think was 87 octane fuel. Back home, you can’t even get fuel with that low of an octane rating, and you’d almost certainly never use it in a turbo car.

Here in NZ, the lowest grade/octane fuel you can buy is 91 for ‘regular’. Premium is 95, and then you can buy super premium in 98, and even 100+ from one specific retailer (which I use in my Suzuki Swift Sport turbo).

However, when you look at an American “gas” pump, you’d be forgiven – from an outsider’s perspective – for thinking that cars in the land of the free are run on crude oil, with octane ratings as low as these:

But why is US fuel low octane compared to what you see in Europe, Australia, New Zealand etc?

Does this mean that the engines fitted to cars sold in the United States are more “agricultural” if they take a lower grade fuel? How does this work with cars built for other markets, e.g. JDM cars imported in to the United States?

Surely there must be a good explanation … that’s why in this edition of Car Facts I’m going to explain why US fuel is lower octane than in other countries.

It’s All An Illusion (Or A Matter Of Mathematics)

The truth is that American fuel isn’t really lower octane.

It’s just an illusion … or more accurately a by-product of how the octane rating of American fuel is measured compared with other countries,

We all know that Americans like to measure things their own way. You only have to look at a global map of countries that use metric versus imperial measurements to know this!

It’s not just restricted to keeping the metric system down. The octane rating of petrol/gasoline is calculated differently in the United States as well.

The United States uses a fuel octane measurement system called the ‘AKI’ or ‘Anti-Knock Index’, also known as ‘DON’ or (R+M)/2. The same measurement system is also used in Canada and some other countries like Mexico.

Many other countries – where fuel appears to have a higher octane rating – use something called the Research Octane Number (RON) alone. I’ll explain more shortly about the differences between these measurement systems.

However, at the most basic level, what you need to know is that American fuel isn’t actually lower octane when you compare “grade for grade” (e.g. comparing regular petrol) it’s simply that the measurement system used in the American market yields different results.

The confusion comes from the fact that both of these different measures are referred to in the general context of being the ‘octane’ rating’ of fuel … but the calculation methodologies are different meaning it’s not an apples-with-apples comparison.

Why Does America Use A Different Fuel Octane Calculation?

I don’t actually have a good answer for this.

I did a bit of digging online and couldn’t find any plausible answer as to why the United States (and a few other countries, primarily in the Americas) use the AKI method instead.

Perhaps the most plausible answer I did find was someone on Quora.com, who claimed that because the United States was an early, heavy user of gasoline – which was often poor quality in the early days – a more robust fuel quality measurement system was required.

The United States is well known for doing things its own way, as the graphic above showing the use of the imperial system indicates.

If you know the actual reason why the AKI octane measurement system was introduced, I’d massively appreciate if you could leave a comment at the bottom of this article.

Understanding RON & MON Calculations

Research Octane Number or ‘RON’ was one of the early methodologies for determining a fuel’s octane rating, and has stuck around since the late 1920s.

At a basic level, fuel is run through a test engine with a variable compression ratio under controlled conditions, and then compared to a “control” measure of iso-octane and heptane to determine the RON octane rating. The compression ratio of the testing engine is gradually increased until knocking occurs, which yields the RON rating.

Fuel with a higher RON rating can withstand more compression before igniting.

MON (Motor Octane Number) uses a standardised set of equipment and testing processes – just like RON – but the test is designed more to simulate how fuel behaves at full-throttle range and higher engine loads, whereas RON is more about simulating engine idle.

What’s most important to understand about the MON rating is that it on a ‘1-100’ scale it typically yields much lower numbers than the RON rating of the same fuel. This is important to understand when it comes to looking at the “AKI” measure/formula that is used in the United States.

This short video by Engineering Explained does a comprehensive job at explaining how the different measures are calculated:

How Is AKI Measured Then?

AKI isn’t actually a unique measurement method, compared with measuring RON and MON that use specific equipment and test procedures.

AKI is simply a mathematical calculation based off the simple mean/average of a given fuel’s RON & MON ratings.

This is why it is often referred to – and listed on fuel pumps – as the (R+M)/2 method, as it is calculated on the basis of adding the RON & MON ratings of a given fuel and then diving that number by two.

If you look closely, on many American fuel pumps you will see the (R+M)/2 formula listed as the octane calculation method.

Because the MON rating of any given fuel is typically several points lower than the RON rating, when you average the two this gives a lower number (a bit like if you developed some kind of length measurement formula that took the average of a length expressed in both inches and centimetres, it would yield a much lower number than centimetres alone).

Further compounding matters, there are actually other octane measurement systems in existence … but you’re unlikely to encounter these in the “real world”.

Recap – Why Is American Gas/Petrol Lower Octane Than Elsewhere?

To recap, it isn’t. It just looks like it is because the calculation methodology is different.

It’s like me saying I am six foot tall – or – 182 centimetres tall.

It means the same thing, and then end outcome is the same, but the calculation methodology is different. One is an imperial measure, one is a metric measure.

On an American fuel pump, the headline octane figure (e.g. 87 for regular) is calculated by the Anti-Knock Index method, also known as the (R+M)/2 method, which takes the simple average of RON and MON.

On European, Australian and Kiwi fuel pumps (and in most other markets) the octane rating is an expression of the RON rating alone of the fuel.

The “base” grade of gas in the United States (regular – ~87) is typically equivalent to the base level in other countries, e.g. here in New Zealand it would be equivalent to regular 91.

Therefore, if you’ve imported a JDM car into the United States that requires 95 petrol, that roughly equates to premium/91 for the American market.

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