$$$ Tip: Don't Buy Premium Gas

Gasoline is typically sold in three grades - regular, mid-grade, and premium - that basically differ only in their octane ratings. Typically these are 87, 89, and 91 octane, but this can vary around the country. So, the obvious questions: Why are these different grades offered and what is their purpose? And should I consider spending the extra money for premium gas?

To eke out maximum performance, upscale and high performance vehicles use higher compression engines. Higher compression requires higher octane gasoline to prevent knocking and pinging that's not only annoying, but can seriously damage engines if unchecked. Knock - technically called pre-ignition - occurs when the compression of the fuel and air mixture explodes without help from a spark plug. Higher octane fuel can better tolerate the greater pressures in high compression engines. Incidentally, turbocharging and supercharging also effectively increase compression ratios.

Most filling stations sell three grades, but some offer a few more choices between regular and premium. However, only regular and premium are typically delivered to the station with the other octane ratings created by blending these two at the pump. This means only two underground storage tanks are needed.

FOR MOST, REGULAR IS FINE
With premium costing 20 to 40 cents more per gallon than regular, there's a growing interest in whether drivers can forego the premium fuel they're accustomed to using in their cars, especially if an owner's manual recommends premium fuel. The greatest concern is whether using a lesser grade will damage an engine, and that's a good question. The key word is 'recommends,' which really means it's okay to use regular under most driving conditions. Virtually every expert - even those from oil companies and auto manufacturers - say that using regular will not harm engines, but you'll probably not get the 'advertised' performance. In other words, you may add a second to 0 to 60 mph times. However, if you're still driving pedal-to-the-metal you're probably not all that concerned about fuel costs, anyway.

Even manufacturers like Porsche, Volvo, Mercedes-Benz, and others that design all their engines to run on premium for optimum performance say that using a lower octane fuel should not cause problems. After all, these cars are sold in parts of the world where high octane gasoline is not always available. For this reason, using regular should not void warrantees unless, in the rare case, the manual 'requires' premium.

Why is this? Modern engines with electronic fuel injection, computerized engine management systems, and variable valve timing can rapidly adjust to different octane ratings to prevent knocking. For example, antiknock sensors determine engine knocking so the ignition timing is automatically adjusted to eliminate this condition.

WHEN PREMIUM SHOULD BE USED
If you own a vintage muscle car or a vehicle that's older than 15 years and made before electronic fuel injection, engine management systems, and knock sensors - and the owner's manual specifies premium fuel - then our recommendation doesn't apply. You will still have to use premium or adjust the timing manually to prevent the knocking and pinging that could occur, for example, when climbing grades, rapidly accelerating, or driving at higher altitudes to prevent engine damage.

Also, you probably should fill up with premium, if recommended, if you plan to carry or tow a very heavy load in hilly terrain. If your vehicle has a supercharger and you drive aggressively, you might also want to use premium. In this latter scenario the knock sensor cannot sense knock fast enough because the supercharger boosts pressure so rapidly.

OTHER RECOMMENDATIONS
Using premium in a vehicle that only needs regular is a complete waste of money. It won't add to performance since the engine is not designed to make use of the higher octane. If your lower compression engine is knocking or pinging, rather than masking the problem by using a higher octane gasoline, get a tune-up. It could be cheaper in the long run if it prevents engine damage and it will make your engine run more efficiently, which will then use less fuel.

One commonly believed myth is that premium gasoline does a better job of keeping an engine clean. However, refiners now put detergents in all grades, not just premium ones. Indeed, a certain level of detergents is required in gasoline by law. Some refiners put in more detergents and advertise additives like injector cleaners, but the benefits are probably minimal and they're usually found in all the refiner's grades.

Finally, some drivers buy a half tank of regular and half of premium because a bit more octane makes sense to them as a way to cure a minor pinging problem. That's what you get by buying a mid-range fuel, anyway.