Premium vs. Regular Gas: How to Decide

Nov 14, 2001 (Updated Nov 16, 2001)

The Bottom Line Most cars don't need premium, but a few do -- and not necessarily the ones you'd expect. And premium's not likely to "clean" your engine, either.

While many people claim that their car runs better on premium, this costly fuel is really only needed in a few circumstances. However, when it is needed, it must be used to spare your engine from costly damage. On the other hand, if you burn premium when your car doesn’t need it, the damage won’t be to your engine but only to your pocketbook.

It’s All In The Numbers

The AKI (anti-knock index -- also known as "octane rating") of the fuel is an average of two different methods of computing the fuel's ability to resist engine knock. Regular unleaded gas in the U.S. has an anti-knock index of 87, while mid-grade is usually 89, and premium typically is 91 - 93. The higher the number, the more anti-knock protection the fuel offers.

Engine knock occurs when the fuel in the cylinder ignites by itself before the spark plug ignites it. The technical name for this condition is "pre-ignition." It sounds like marbles rattling around in a can, and it generally gets worse the harder you press on the accelerator pedal.

Knocking can indeed damage an engine because what is in effect happening is the "explosion" of the fuel/air mixture is trying to push the piston downward in the cylinder before it's gotten to the top of its stroke and is free to move downward. The engine is actually working against itself to a degree, and there is a lot of mechanical stress placed on certain engine parts, such as the pistons. In extreme cases, knocking can burn holes in the pistons and create other forms of engine damage.

Oil companies sell higher AKI fuels (also known as premium) to address this problem. A fuel with a higher AKI actually burns more SLOWLY than fuel with a lower AKI. This is because the fuel is less volatile and requires more heat input before it begins to ignite.

(For this same reason, high AKI fuels will actually make your car HARDER to start on a very cold day. In extreme cold -- below 0 degrees F -- you want a more volatile fuel which ignites easily so you can get the engine going without extensive cranking. So if extreme cold is expected, buy 87 AKI gas unless your owner’s manual advises against it.)

Who Needs Premium?

Basically, all premium fuel does is resist knock. So the short answer to the above question of “Who Needs Premium?” is simply any vehicle whose engine knocks without it. Consider these guidelines as you decide whether you really need premium or are in the majority who do not:

1. If your vehicle makes the above-described knocking sound when you accelerate, try the next higher grade when you refuel, and then see if the knocking goes away. If it does, stick with that grade. If not, bump it up another grade the next time you get gas. Generally speaking, high mileage vehicles with a lot of carbon deposits in the engine are likely candidates for needing premium or mid-grade gas, since these deposits reduce the volume of the engine's combustion chamber and make knocking more likely. In addition, the carbon deposits themselves get hot and may act to ignite the fuel if lower AKI fuel is used. High mileage engines do not always have carbon deposits; those that have been driven mainly in town and done a lot of idling are most likely to be “carboned up.” My Ford F-150 has 210,000 miles on it and does not knock on 87 AKI regular unleaded, so that’s what I use.

2. If your owner's manual calls for higher AKI fuel, use what the Good Book says. Usually this will be specified by manufacturers of high compression, high performance engines. If lower AKI fuel were used in these engines, performance would suffer because the engine's computer system would have to retard the ignition timing (reducing horsepower and fuel economy) to keep the engine from knocking.

If, in such an engine, the computer could not retard timing enough to keep it from knocking, the engine itself could be damaged. This would not be likely to happen on one tank of fuel, however. So if you lend your BMW to your son or daughter who has a hot date, don’t run for the siphon hose if he or she returns it with a full tank of regular unleaded. Just drive more easily than normal so the engine's maximum performance isn't needed, and there should be no problem.

But I Want to Keep My Engine Clean!

The higher AKI of premium gasoline does not in itself make it any cleaner than regular gas. Oil companies, however, like to advertise that their premium fuels are ‘specially formulated” to clean fuel injectors, restore lost power, and, hey, maybe even improve your sex life. But the truth of the matter is that any “cleaners” that could be added to gasoline itself are probably not going to be present in sufficient quantities to do much good.

A better strategy to keep your fuel injectors clean is to buy your gas from reputable stations that have a relatively brisk business. In this way the gasoline is liable to be freer of water and other contaminants. It’s also not a bad idea to avoid buying gas from stations while their tanks are being filled by a big tanker truck. It’s possible that the filling process could stir up gunk that’s settled to the bottom of the station’s storage tanks and make it more likely that debris could wind up in your own tank. (Gas stations have filters that are supposed to prevent this, so this is a minor point for those who want to be extra careful; it’s not a really big deal.)

“But I Just Think Premium Makes My Car Happy”

If you enjoy using premium because you think it makes your car “happy,” then go right ahead and use it. President Bush has asked Americans to spend money to support the economy, so you can view it as your small patriotic contribution. And if you own stock in an oil company, so much the better. Using fuel with a higher AKI than needed will in no way harm an engine. Also, your engine won’t get “a taste” for the good stuff and rebel against you if you fall on hard times and have to use cheap gas.

Alcohol: The Tough Question

No, this isn't a lecture on drinking and driving. Ethanol is a type of alcohol that is often added to fuels to increase the AKI rating. Government regulators like it because they claim it helps engines burn cleaner. Should you use it?

This is a more complex question than what AKI rating to use. Alcohol-gasoline blends may make a car do better on an emissions test, so if you need one to get your car licensed in your state and are afraid your car may not pass, fill your tank with an ethanol blended fuel before you take it for its emissions test.

Whether these blended fuels actually keep any pollution out of the environment is not as certain. This is because emissions tests don’t necessarily take into account the fact that your car may burn more fuel if you use an ethanol blend. On a per-gallon basis, a car will emit fewer pollutants with an ethanol blend fuel because ethanol burns more cleanly than gasoline. But on a per-mile basis -- the only basis that matters -- an ethanol blend fuel might not be any cleaner than gasoline because cars typically get worse gas mileage with it than with straight gasoline.

For example, if you get 30 miles per gallon with straight gasoline, and 28 miles per gallon with an ethanol blend fuel, for every 1000 miles you drive, you are burning a little over two more gallons of the ethanol/gas blend than you would have burned if you had used straight gasoline. So even if the ethanol/gas blend burns, say, 5 percent cleaner than straight gas, you are still actually polluting more with the blend because you’re burning almost 7 percent more fuel.

There are other issues as well: Ethanol has a tendency to degrade rubber fuel system components such as gaskets. While late model cars have supposedly been made with gaskets that resist damage from ethanol, some older models may not be so resilient. What can happen is little pieces of rubber that have come off of deteriorating gaskets can find their way to fuel filters or fuel injectors and gum things up.

All in all, for the environment as well as most vehicles, ethanol in gas is probably not going to do much harm or much good. But on the off chance that it may, I personally prefer to avoid it. Trouble is, in a lot of states, stations do not have to tell you if their gas has ethanol in it. You can, however, look for stations who advertise their fuel as “100 percent gasoline” or similarly.

(These comments do not apply to newer dual-fuel engines that designed to run on either gasoline or methanol. If you have one of these engines and use methanol, follow owner’s manual recommendations.)

In Summary

Fuels with a high AKI number are needed in a few cars, but the majority will run fine on regular unleaded. If your car knocks when you accelerate, or if your owner’s manual calls for a higher AKI, use one of the premium grades. Otherwise, regular unleaded will save you money and reduce your driving costs.

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