Drive Time

Next Time, Fill 'er up With Regular

Gas prices are spiking again. Nationwide, the average cost of a gallon of regular is back above $2, according to AAA, and a gallon of premium is $2.21.

So let's say you drive a Cadillac SRX with a V8 engine and, per the owner's manual, you spring for premium gas each time you need a fill-up. If your 20-gallon tank is almost empty, each time you fill up, you spend $44. In the course of a year, assuming you drive 15,000 miles, you're likely to spend $1,950 just on gasoline.

Now let's say you filled it up with regular instead of high-test. You just saved yourself $200 a year. And except in rare cases, you're not going to hurt your engine by being thrifty at the gas pump.

Don't pay a premium

It seems the more expensive the car, the more it costs to maintain. Of the thousand-plus 2005 models listed in the EPA's latest fuel-economy guide, about a third -- most of them luxury or sports cars -- recommend premium.

In fact, most expensive cars are designed to run on premium gas -- fuel with an octane rating of 91 or more -- to maximize their performance. That's because high-performance engines have a higher compression ratio.

Translation: Gasoline and air are drawn into the cylinders and compressed as the piston pushes up. As the spark plug ignites, the mixture expands, which drives the pistons back down and produces power. If the mixture sparks too soon, it leads to uncontrolled burning in the combustion chambers, which you hear as engine knock.

Engines with a high compression ratio are more susceptible to uncontrolled burning with lower-octane gasoline. High-octane gasoline is more resistant to knocking because it's harder to ignite.

Most cars can adjust to regular fuel because engines are now equipped with knock sensors, which adjust the engine's timing automatically when they detect uncontrolled burning. You may experience a slight decrease in power, but no damage to the engine.

One caveat: Supercharged engines often do need a higher octane gasoline because such engines boost pressure too fast for the knock sensors to work properly. For example, for its 390-horsepower, supercharged engine, Jaguar recommends 95-octane gasoline (though you're unlikely to find higher than 93 or 94).

Also, you could lose some fuel economy if your engine is designed for premium and you use regular instead. It's probably not enough to pay the premium for premium gas, but do your own tests with both grades of gasoline to be sure.

The premium myth

One thing's for sure -- if your owner's manual doesn't recommend premium, you're wasting your money if you use it. You simply can't buy extra performance.

In fact, because it's harder to ignite, the higher-octane gas could make your car harder to start and run less smoothly, especially as the weather turns colder.