85 Octane Regular?


I recently moved to a state where “regular” gasoline is 85 octane. What gives? Is it the same as 87 octane in other states and just a numbers game? Most cars are supposed to use 87, so how can a state sell 85 as regular? Any explanations?


You probably moved to a state like Colorado which is at a higher altitude than where you had been before. The octane does not need to be as high at higher altitudes. The octane is the resistance of the gasoline to burning–the higher the octane, the higher the ignition point. The fuel burns more slowly at the higher elevations, so the octane can be reduced.


When you are climbing a hill and your engine starts pinging, have you ever noticed that the pinging goes away when you let off of the gas a little? That’s because the intake manifold pressure gets reduced (higher vacuum) and the air ends up getting less compressed at the top of the compression stroke. Because the ambient air pressure is low at high altitudes, there is less pressure in the cylinder at the end of the compression stroke and you need less octane rating.


There’s a lot of different ways of saying basically the same thing. I look at it this way: Octane is the fuel’s resistance to spontaneous ignition due to compression (like in a diesel). The higher your car’s compression ratio (i.e. the volume of the engine with the piston down divided by it’s volume all the way up) the higher octane you need. If you’re starting out with fewer air molecules due to lower air pressure though, there’s fewer air molecules actually getting squashed, more space for them with the piston all the way up, and therefore they’re less likely to preignite and you can get away with 85 octane!

Of course, the real reason is that they sell whatever the government lets them sell, which isn’t necessarilly a direct function of altitude. You’ll see 85, 85.5, and 86 octane all over mountain country, and I’ve at least never noticed a correlation between these octanes and altitude. Sometimes you’ll see 85 at one pump and 86 across the street.