# Why does premium octane-level vary country wide

**URL:** <https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554>\
**Category:** Factual Questions\
**Created:** [October 4, 2004, 10:48pm UTC](https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554 "2004-10-04T22:48:59Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![qubed](https://avatars.discourse-cdn.com/v4/letter/q/f07891/32.png) [@qubed](https://boards.straightdope.com/u/qubed)\
**Post date:** [October 4, 2004, 10:48pm UTC](https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554/1 "2004-10-04T22:48:59Z")

</div>

Back in eastern New Mexico (in fact, in the heart of oil country), finding premium gas with an octane lever \> 90 is virtually impossible. I’ve never found it in my hometown. In fact, my parents own a car that suggests using an octane-level of 91, but they’re forced to use only 90 octane-level gas (which i’m hoping is not that big of deal…)

But if I cross the border to Texas, it’s regularly 92 and sometimes 93.

Here in California, Bay area, I can’t remember exactly the numbers, but IIRC, 91 or 92 seems to be the norm.

And even in Europe, unless I was decieved, I often saw levels of 95 and higher. In fact, there was generally only one level of octane there - but apparently that’s due to different rating systems.

---

<div class="post-metadata">

**Author:** ![Mr.Niceguy](https://avatars.discourse-cdn.com/v4/letter/m/ce7236/32.png) [@Mr.Niceguy](https://boards.straightdope.com/u/Mr.Niceguy)\
**Post date:** [October 4, 2004, 10:52pm UTC](https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554/2 "2004-10-04T22:52:51Z")

</div>

There are minimum octane ratings set by by the authorities having juristiction over your location. They vary from state to state and country to country.

---

<div class="post-metadata">

**Author:** ![chukhung](https://avatars.discourse-cdn.com/v4/letter/c/e79b87/32.png) [@chukhung](https://boards.straightdope.com/u/chukhung)\
**Post date:** [October 5, 2004, 12:16am UTC](https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554/3 "2004-10-05T00:16:31Z")

</div>

Another thing that could be a factor is the elevation of where you buy the gas. For example, “regular gas” sold above about 5000 feet ASL may be only 85 octane or less (and the higher grades are shifted downward as well).

This [page](http://hmchemdemo.clt.binghamton.edu/zumdahl/docs/chemistry/03stoichiometry/library/03octane.htm) says

> [@](#):
>
> A consumer typically is faced with three choices of gasoline, with octane ratings of 87 (regular), 89 (midgrade), and 93 (premium). But if you happen to travel or live in the higher elevations of the Rocky Mountain states, you might be surprised to find different octane ratings at the gasoline pumps. The reason for this provides a lesson in stoichiometry. At higher elevations the air is less dense-the volume of oxygen per unit volume of air is smaller. Most engines are designed to achieve a 14:1 oxygen-to-fuel ratio in the cylinder prior to combustion. If less oxygen is available, then less fuel is required to achieve this optimal ratio. In turn, the lower volumes of oxygen and fuel result in a lower pressure in the cylinder. Because high pressure tends to promote knocking, the lower pressure within engine cylinders at higher elevations promotes a more controlled combustion of the fuel-air mixture, and therefore, octane requirements are lower.

---

<div class="post-metadata">

**Author:** ![Gaudere](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@Gaudere](https://boards.straightdope.com/u/Gaudere)\
**Post date:** [October 5, 2004, 12:36am UTC](https://boards.straightdope.com/t/why-does-premium-octane-level-vary-country-wide/267554/4 "2004-10-05T00:36:28Z")

</div>

[Moderator Hat ON]

I think this will do better in General Questions.

[Moderator Hat OFF]