Grades of Gasoline

General Discussion
1

A loyal reader of the Wisconsin State Journal in Madison, WI wrote in asking:



I live in the mid-west where our grades of gasoline are rated 87, 89 and 92 octane. My car’s owner’s manual says to use 87 octane, the so called regular grade. When I travel to Colorado and Utah, the octane ratings are 85, 87 and 91. What is going on? The regular octane is apparently 85 there. Is this related to the higher altitude? Is it OK to us 85 octane when I travel there?



Thanks

Bill Simpson

2

Several points:

You manufacturer said to use XX octane. (Did the recommend it or require it? In the first case using the lower octane will reduce you mileage and power but not likely much. In they said you must use the higher octane, you can add possible engine damage to the above negatives.

If they did not say anything, then there would be no advantage to higher octane fuel. If not at least recommended, it will not damage the engine, lower mileage or lower power.

Note, at high altitude your car’s octane change and you don’t need as high an octane.

3

I concur with Mr. Meehan, but I want to insert a word that he omitted from his last sentence:

“…at high altitude your car’s octane requirements change, and you don’t need as high an octane.”

4

The difference between octanes is how much they resist spontaneously combusting from compression alone (like in a diesel). If you have a car with a higher compression ratio, the fuel is more likely to spontaneously combust and this is why you need a higher octane.

When you get into higher altitudes the engine is drawing in fewer air molecules for a given volume of air and so when in compresses them on the compression stroke there’s less pressure and the fuel is less likely to spontaneously combust. So you can use a lower octane fuel.

You should still read your owner’s manual-- some manufacturers require the same octane at high altitude, especially on turbocharged cars.

5

Octane demand drops as elevation increases. The oil companies reduce the octane of the gasoline sold in high-altitude areas, but not the price…Your car will accept the 85 octane fuel as long as you are above 4500 feet elevation…

85-87, that’s worth about .12 cents…I wonder where that money goes…

6

That’s not really true in my experience. They sell 87 octane regular here, but in a comparably-sized city about 100 miles to the northeast they sell 85.5 or 86 and it’s usually about 5-7 cents cheaper for the same brand. Since the mid-grade is 88 octane, I think that works out to your cost-per-octane number being the about the same as in 87 octane-land.