# Octane Ratings **URL:** \ **Category:** Factual Questions\ **Created:** [May 3, 2000, 2:57am UTC](https://boards.straightdope.com/t/octane-ratings/18422 "2000-05-03T02:57:44Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [May 3, 2000, 2:57am UTC](https://boards.straightdope.com/t/octane-ratings/18422/1 "2000-05-03T02:57:44Z")
On gas pumps, there’s a sticker that has the octane rating of the gasoline. It also usually says that the octane rating is determined by the (R + M) / 2 method. What the heck are R and M? For that matter, what exactly is an octane rating? I know that “octane” is a specific kind of hydrocarbon, but does an “octane rating” of 85 mean that the gasoline is exactly 85% pure octane? ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [May 3, 2000, 3:41am UTC](https://boards.straightdope.com/t/octane-ratings/18422/2 "2000-05-03T03:41:07Z")
The octane number measures nothing more or less than the anti-knock property of the fuel. Heptane is defined to have an octane number of zero. 2,2,4-trimethylpentane (an isomer of octane) is given a rating of 100. An 87-octane gasoline has the same anti-knock properties as a mixture of 87% iso-octane and 13% heptane. A gasoline need not have any octane in it to get a rating of 100. (In practice I’m sure all gasoline does contain octane). I believe it’s possible to to have an octane rating higher than 100. High octane ratings do not necessarily mean higher energy content. ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [May 3, 2000, 3:45am UTC](https://boards.straightdope.com/t/octane-ratings/18422/3 "2000-05-03T03:45:58Z")
By the way, R and M refer to 2 different ways of calculating the octane rating. See [Fuel Ratings](http://www.earlham.edu/~chem/pages/gasoline/grades.html) which also explains how they calculate octane ratings over 100.