Do I have to pay premium prices for premium gas if I live at high altitude?

Dear Car Talk

Dear Car Talk | Jun 01, 1997

Dear Tom and Ray:

My owner's manual says I should use only "regular unleaded gasoline rated
at 87 octane or higher." However, I live in Colorado and use 85 octane.
The engine does not ping or knock at any time. Since the price of 87
octane gas here is about 10 cents more per gallon, do I have to spend the
extra money for it? I own a 1995 Buick Regal with a 3800 V6 under the
hood. -- Bob

RAY: Well, first of all, I'm glad you have a 3800 V6 under the hood, Bob.
Because if it was anywhere other than under the hood, this question would
be a much greater challenge for us.

TOM: You're the exception to the octane rule, Bob. For those of you who
are not regular readers (for those who stumbled onto this column by
accident while looking for something more interesting to read), we always
say to use the manufacturer's recommendation for octane. No more, and no

RAY: Lower octane can cause pinging and overheating, which can damage the
engine. And higher-than-required octane is simply a waste of money.

TOM: An octane rating tells you how high the ignition point of the
gasoline is. If the ignition point is too low, the fuel can begin to burn
too early in the cylinders and cause pinging. By using higher octane gas,
you raise the ignition point and eliminate that early burning.

RAY: And the manufacturer determines what octane you need based on the
compression ratio of the engine. Generally speaking, the higher the
engine's compression, the higher the octane you need.

TOM: And what allows you, Bob, to use lower-than-recommended octane is
the altitude in Colorado. Up where you live, the air pressure is lower.
That means the pressure in the cylinders is lower, too. So by going up
5,000-plus feet in elevation, you've effectively lowered the compression
of your engine, and that means you can use lower-octane fuel without
making the engine ping.

RAY: At the altitude where you live, 85 octane is the equivalent of 87
octane at sea level. So, by all means, go ahead and use it, Bob. Along
with getting winded more easily, it's one of the fringe benefits of
living up there in the mountains.

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