Octane Question

What does this mean in a car’s manual under Fuel: “87 Octane (Research 91)”? When I searched Google there were inconsistent answers. The answer given by 2 sources was that 87 octane is a U.S. rating and “Research 91” is used in other countries and is the same as 87 octane. Is that correct? The way it is written is confusing.


Since the US could not choose an octane rating method… Research (RON) or Motor (MON) for rating gasoline, they decided to average the 2. RON always gives a higher number and MON a lower number.

So 87 US octane rating is the same as 91 (RON) research.

Thank you! It should be explained better in the manual. Or maybe I’m just naturally confused… :grinning:

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Good heavens no. That would just confuse more people, and probably give even more people reasons to NOT even open their manuals. The manual should just state what is required/recommended and that damage may occur by not using what is recommended/required. Period.


Agreed. No need to explain. Most people have no idea what Octane means anyways. From my personal observation well over half car owners think higher octane think it’s higher quality -mainly because advertising by the gas companies.


Both of our vehicle say 87 Octane . Push 87 button on pump , fill up and find something important to worry about . Like why can’t I reach the first hole Par 5 in 2 anymore .


There seem to be two theories about how to present technical information. One is to error on the side of under-explaining; i.e. condense to be as brief as possible. The other is to error on the side of over-explaining. Either way can cause confusion. I prefer the latter myself, but do find over-explained prose sometimes to be very confusing. I think most auto shop techs prefer the under-explain method, b/c over-explaining often results in even more time-consuming questions from the customers.

I always thought “iso-octane” had 7 carbons in a row, and just one branch carbon. Apparently from the diagram above, that’s incorrect. That’s what happens when I do my own thinking … lol …

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Your curiosity is commendable but from practical perspective, Toyota not only recommends it but its vehicles run just fine on 87.

Just to add some confusion, not on my part but others may be confused. On the fuel door of my 2008 BMW 650i

So I look in the manual:

Hmmmmmmm…… is 91 required, highly recommended or not even necessary as the manual says 87 is the minimum acceptable.
P.S. I just use 91, the extra $8-$10 to fill up ain’t going to kill me.

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If you used 87, the car would probably respond to the knocking by retarding the timing, resulting in worse fuel economy, in which case the actual difference would be less than $8 to $10.


I ALWAYS over-explain. That’s how we engineers roll!

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