Posted by: Rob | July 13, 2008

The Fallacy of the Gasoline Octane and Mileage Correlation

****UPDATED: Note that this post has been revised.  I’ve left the original comments, though, but the context may be a little confusing.

When I blog I’m usually either relating some real life event or events (often slightly modified for dramatic effect or expedience) or providing my valuable opinion on some topic.  If I invoke something that could be construed as factual, I go to some length to ensure the veracity of the information I’m parroting or publishing.

Recently, I stumbled onto this post via the WordPress dashboard.

Because of my background in fuels refining and my ongoing pet peeve with the fact that the majority of people who talk about fuels related topics know little to nothing about it, I was interested to see what the writer had to say about this.

I went to the blog entry and read the brief piece.  I also went to the two posted links on the piece and read those.  I also read the comments that the writer had (which included at least two spam comments).  And I experienced a brief feeling of “Here we go again.”  I had to leave a comment. 

Regular readers and bloggers where I comment will already know that, at times, my comments are not brief.  That much was true in this instance.  Since not everyone (like regular readers – all two or three of you) would get to benefit from my wisdom, I thought I would reproduce my comment here.

Rob’s Comment:

The level of incorrect, erroneous and dis-information on the internet really never ceases to amaze me.

Like the lack of a real correlation between crude oil market price and motor fuel pump price, there is no relationship between gasoline octane rating and vehicle mileage.

The only correct information in your post is this:

Octane does not refer to the energy level of a fuel but to its resistance to auto-ignition or detonation.

The same is true for the content of the two links you published.

Octane rating is a measure of the burn characteristic of the fuel, primarily the speed at which the flame front initiated by the spark plug moves as the fuel burns. Too slow and you don’t get the energy from the fuel (no power); too fast and you have detonation (engine pinging or knock).

Interesting that you didn’t explain the part about the (R+M)/2 aspect of octane rating. If you don’t know, the “R” is for Research and the “M” is for motor. These are individual octane ratings – determined empirically at the refinery lab using one cylinder “knock engines” (that are subject to standardization testing and compliance) – that reflect average engine operation and performance at two ends of the operating spectrum. The resultant figures are numerically averaged to provide the number posted at the pump.

Really, octane rating is more or less the common specification arrived at between fuels refiners and automakers so that the cars and trucks made by the automakers will run on the fuels made by the refiners.

Motor fuel is, of course, derived from crude oil (petroleum), a commodity that is extremely variable in nature depending upon its source.

The refining process started out as the simple distillation of crude oil in fractions but, over the years, has evolved into a more complex upgrading process involving several processing steps of the various fractions before they are finally blended into the fuels provided at the pump.

Because there is no easy way to quantify motor fuel by component, simple distillation analyses, octane ratings and a few other measures, like Reid Vapor Pressure (RVP) are used to ensure a given fuel meets commodity specifications. (And that makes the comment about the loyal Chevron customer laughable, because fuels – as commodities – are swapped for transportation expediency and you can never know for certain if the fuel brand you are buying was actually manufactured at a refinery owned by that brand.)

There are seasonal variations on gasoline specs as well, especially in more northern latitudes, primarily in the RVP. Summer gasoline won’t vaporize in a cold northern winter, so refiners dispose of n-butane by blending it into the gasoline in the winter when RVP limits are higher (this doesn’t work in the summer because the n-butane will all vaporize into the atmosphere and EPA doesn’t really like that. Mind you, neither would you – imagine paying for gasoline that’s flashing off into the air before it gets into your tank?) By the way, n-butane in winter gasoline is part of the reason why your mileage suffers in the winter – it isn’t just excessive idling.

The bottom line if you want better mileage is to change your driving habits:

1) Slow down. Driving 50 – 55 mph will save you quite a bit of fuel vs. driving 65 – 70 mph.
2) Don’t punch it off the green light. Hard and fast acceleration eats fuel like nobody’s business.
3) Try to avoid stops and starts at intersections. Use gentler speed ups and slow downs (i.e. look ahead) instead of hard braking and heavy on the accelerator.
4) Don’t idle. If you’re sitting longer than 10 seconds in a mall parking lot or whatever, shut the engine off to conserve fuel.

There are other, more draconian things you can do to improve mileage (like not using your A/C) but the above four points, if implemented, will help extend your range out each tank of fuel.

Oh, in case you’re wondering “WTF does this guy know?”, I guess I should add the fact that I’m a chemical engineer who has worked in oil refineries.



****UPDATED: Note that this post has been revised.  I’ve left the original comments, though, but the context may be a little confusing.




  1. (sigh) Your facts? Clearly mucking up this guys rant, Rob!

    What to do? Well, you could always capture the comment on your own blog, with a ‘pingback’ to the author. At least you’d have the satisfaction of knowing you clarified, even if the author didn’t want to be distracted by actual technical data…


  2. While I applaud your quest for accuracy, I am of the opinion that presenting a coherent, logical and factual argument will never be quite as satisfying as saying, “You are wrong. I have all the proof here:” and then Rickrolling the victim individual concerned.

  3. Some people just will not allow the facts to get in the way of what they believe to be the truth.

  4. I just read the link and your comment is up over there. does that mean you were a little hasty in saying it was rejected?

  5. daisyfae: That’s a very good idea. Maybe I should…hey! That’s what I did!

    kyknoord: Welcome to the Tome! Thanks for your comment. But I wish I knew what Rickrolling was…

    UK: The problem I see is that not everyone knows the facts. Partly because bad information takes on a life of its own.

    nm: *Sigh* Yes, perhaps it does mean I was a little hasty. But I’m just spoiled by good bloggers, like yourself, who are very attentive to their blogs and post comments quite quickly. Fortunately, writing slagging blog posts is the only thing I’m hasty with. And besides, good bloggers – like you – always do their research and don’t publish inaccurate or wrong information.

    PS: What happened to your avatar?

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