Am I spending money needlessly on premium gasoline?

Dear Car Talk

Dear Car Talk | Dec 01, 1997

Dear Tom and Ray:

We purchased a '97 Nissan Maxima automatic in January and love it! The owner's
manual recommends "unleaded premium gasoline with an octane rating of at least
91." It also says, "If unleaded premium is unavailable, unleaded regular
gasoline with an octane rating of at least 87 can be used." Whenever I fill up
the car (which is most of the time), I follow the manual's recommendation and
use premium gas. But when my husband fills her up, he uses regular gasoline
even when premium is available. So my question is, am I spending money
needlessly on premium gasoline? And why would Nissan recommend premium gasoline
if regular is OK to use? -- JoAnn

TOM: You're doing exactly the right thing, JoAnn.

RAY: This car has a high-compression engine that's designed to run on premium
gas. But since there are places where premium gas is not available, and because
premium gasolines can vary in octane, most manufacturers, including Nissan, use
a "knock sensor" to protect the engine.

TOM: The knock sensor detects -- that's right -- "knocking" (also known as
"pre-ignition" or "pinging")! Knocking is bad for the engine, and it often
results from using lower-than-recommended-octane fuel. So when your cheapskate
husband puts regular unleaded in the Maxima, the knock sensor kicks into action
and retards the ignition timing to protect the engine.

RAY: Retarding the timing prevents the knocking, but it also reduces the
engine's power, decreases fuel economy, probably increases the emissions, and
may lead to a buildup of residue on the valves. So it's not an ideal situation.
It won't hurt the engine if you use regular gas occasionally, but it prevents
the engine from performing at the specifications at which it was designed to

TOM: So I'd either take away your husband's keys, or increase his allowance by
a few bucks a month so he can afford to buy premium. And if he's unhappy about
having to spend the extra money, remind him that a car's fuel requirement
(which can add many hundreds of dollars to the cost of owning a car over the
years) is one of the things you should find out about before you put down your

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