Why is gas octane lower in mountain states?

[FatBaldGuy]

After posting in another thread about gas prices, I realized I have never got a satisfactory explanation of this practice. In Utah (and apparently also in Colorado) the octane ratings for gas are generally 2 points lower than the rest of the country. Regular gas is 85 octane, mid-grade is 87 and premium is 91, compared with 87/89/93 in the rest of the US.

I’ve always assumed it has something to do with the altitude, and the way engines perform at higher elevations regarding octane levels, but does anyone really know why this is?

[chukhung]

This page, http://hmchemdemo.clt.binghamton.edu/zumdahl/docs/chemistry/03stoichiometry/library/03octane.htm, says

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.

[FatBaldGuy]

Thanks, chukhung! It’s nice to know there really is a scientific reason for this, and that they’re not just ripping us off.