91 Octane

[BYprodriver]

Originally Posted by blue2000s

It's a matter of air density. Because there are fewer molecules of air and fuel to rub against each other in a combustion chamber at altitude, the tendency to auto-ignite is lower. Thus the lower octane number because it's correct for the altitude where it is sold, and the lower octane requirement.

The 91 fuel at the pump in Colorado IS the same fuel as the 93 octane at seal level.

If fuel rated at 93 octane at sea level is used at high elevation (2,500 ft & higher) it may increase the octane rating, but it definitely will not lower the octane rating.