Octane vs Altitude

The Prius has a knock sensor and uses it to tune for the grade of gasoline. I've long had a desire to run 85 octane at high altitudes for some Prius mileage tricks. . . . Perhaps another day.

Bob Wilson

 

OK let start at the beginning.

When the piston goes from the bottom of the cylinder to the top, it compresses the air in the cylinder 5 to 15 times as much as it was when the piston was at the bottom. In general the higher the compression, the higher the octane needs to be to resist spontaneous combustion or Knock.

By the time you are at 4000 feet, over 15% of the air is below you so your air trapped in the cylinder is 15% less and your compression ratio act like it is 15% less.* Because this altitude induced Knock resistance will remain while you are at 4000 feet you need lower octane gas to run well. (If you buy gas at 4000 feet and then commuted to sea level, you would have some Knock. Get better gas when you are about to leave CO)

In theory, you would have the opposite issue 4000 below sea level, but hardly anyone drives gas engined vehicles 4000 feet under sea level.

*turbochared engines do not work like this, as the turbo just keep on pumping air until they get the same density as sea level, do not buy low octane gas for a turbo

 

Ran a tank of 85 octane in Reno Nevada in our 2010 Prius. No problems and the same mpg's as 87 octane, 55+ mpg calculated, with 87 octane or 85 octane.

 

We just drove from FL to Colorado Springs. Regular gas was 87 octane from FL to maybe Ft. Worth TX altitude 610ft. At Childress TX altitude 1900ft, gas switched to 86 octane. It also was 86 octane in Clayton NM at 5000ft, and was 85 octane in Colorado Springs at 6000ft.

We do not know at exactly what altitudes the octane changes, as we only bought gas at Buffalo TX 300ft about 120 miles southeast of Ft. Worth, at Childress, Clayton & Colorado Springs. We did not notice any change in mpg that we could blame on lower octane.

 

Many threads...Prius is fine with 85 Octane in those high altitude areas where 85 is same as 87 due to altitude. We must have officially from Toyota somewhere...not sure where.

 

I agree with most of the above responses. Don't worry about the 85 octane rating if you aren't hearing knock, and aren't about to take that fuel down to a much lower altitude. That's the same as for any other car designed for "regular" unleaded.