All gasolines are not created equal. Today’s gasoline is a highly complex blend of chemicals that varies from brand to brand, region to region and season to season.

Since gasoline was developed during the Civil War, numerous substances have been added to or removed from it to make cars runs better and clean up vehicle exhaust. And that’s precisely the problem. You see ads on television touting one brand or another.


You roll into a gas station and are confronted by at least 3, sometimes even 5, pumps with different grades of gasoline. How is one to choose? What’s the right gasoline for you? Let’s separate fact from fiction so that you can make an intelligent buying decision.

Choosing a grade

Generally, major brand gasoline is sold in various grades ranging from 86 to 94 octane. An octane rating of 87 is considered Regular grade. Anything below that is usually called Economy grade. Midrange octane’s from 88 to 91 are usually called Regular Plus or just Plus. Anything above 92 octane is considered Premium. By law, octane ratings have to be clearly displayed on gas pumps.

So, with up to 9 grades facing you when you pull up to the pumps, which should you choose? Vaughn Burns, Chrysler’s emissions technology manager and top-fuels guru, advises to start with your owner’s manual. It won’t specify a brand—that decision is yours—but the manual will recommend which octane grade your engine requires. Octane rating is a term that describes gasoline’s ability to resist engine knock.

Knock is a type of detonation that occurs after a spark plug fires. Unburned air/fuel mixture in the engine cylinder spontaneously explodes, literally ignited by the heat and pressure created from normal combustion. What you hear from the driver’s seat is a bunch of metallic taps coming from the engine. Those taps are the explosions. This unintended detonation creates harsh vibrations that put enormous stress on the engine and sometimes cause catastrophic damage.

Many trucks are designed to run without knocking on Regular unleaded fuel with an 87-octane rating. Auto manufacturers recommend that high-performance vehicles be filled with midrange 89 octane, or even 92 or 93 octane, to prevent knocking. This information is in your owner’s manual. There may also be a printed message right on the dashboard near the gas gauge that warns, “Use Premium unleaded gas only.” There may be a similar label inside the gas-filler door. If so, you should use only gas labeled “Premium,” “Super” or similar designation. If your car has no such label, your engine will run on Regular.

Burns says that transient knocking should not be of concern to car owners. “If you experience light spark knock, that won’t damage your engine,” he says. But if your engine knocking is persistent, then you need to have your car checked for mechanical problems. Those problems may be developed by deposits that develop on fuel injectors, fuel-intake valves or the combustion chambers in the engine.

If your engine knocks when you climb hills, is difficult to start, idles rough or stumbles and surges, you may be using too low an octane. An engine that stalls, or runs on when you turn the ignition key to the OFF position, may also be the result of using too low an octane. Sluggish acceleration is another sign of too-low octane.

One reason for this, says Eddie Green, Shell’s manager of automotive fuels technology, is that today’s vehicles are equipped with electronic knock sensors that alert the engine-control computer if the gasoline is too low in octane for the type of driving you’re doing. It automatically adjusts the engine to prevent damage, but the adjustments greatly deteriorate engine performance and fuel economy.

Jerry Wilkins, Sunoco's manager of fuel product development, says that his company sells five octane grades: 86,87,89,92 and 94. Tanker trucks actually deliver only 86 and 94 octane gasoline to Sunoco stations. To dispense the three mid grades of fuel, the pumps blend together appropriate amounts of 86 and 94.

Why does Sunoco sell 86 octane fuel when carmakers recommend 87 octane as a minimum requirement? The 86 octane gas is less expensive to refine and can be sold cheaply.

Jim Nordmann, manager of technical analaysis for Volvo Cars of North America Inc., says his company recommends using a higher octane fuel in most older vehicles if the engine begins to knock. However, Gary Smith, manager of service technology for Toyota Motor Sales U.S.A., says, "if we design a car to run on 87 octane, we fully expect that for the lifetime of the car it will run on 87 octane fuel." He adds that if the vehicle doesn't run well on gas with the specified octane rating, there's something wrong with the engine and it should be repaired. 

Nordmann also suggests stepping up to a higher octane fuel if you do a lot of high altitude driving or steep hill climbing. In addition, Volvo recommends high octane gasoline exclusively for turbocharged engines.

What if you're low on fuel and a service station doesn't have the high octane gasoline your car requires? Go ahead and fill up with regular gas, Wilkins advises. Then go back to using a higher octane with the next fill up.It would probably take at least 20 consecutive tankfulls of the wrong gas before you might damage an engine, he says.

By the way, there's an old wives' tale about burning up your engine if you use a gasoline with too high an octane. No way. There is no such thing as too high an octane. Your engine may not be able to use the potential antiknock capabilities of the high octane gas, but it won't cause any harm whatsoever- except to your wallet, since high octane gas can be much more expensive per gallon than Regular.

Additives and Formulations

There's no uniformity in the way oil refiners blend additives into gasoline. That's why selecting the optimum type to burn in your vehicles engine is not always a simple decision.

"We spend millions to make sure gasoline has correct driveability and cleaning formulations," says Shell's Green, adding that formulations are tested to make sure gasolines operate in all kinds of driving and weather conditions. Additional testing is done to ensure the gasoline will not leave harmful deposits on internal engine parts like intake valves, fuel injectors or cylinder walls-which can cause knock and affect octane requirements.

Other substances have been added in minute quantities to improve gasoline's ability to ignite and burn completely and to prevent fuel residues from forming deposits that could damage engines. Additives include detergents, corrosion inhibitors and substances that are blended in during different seasons to make sure your car starts easily in winter and doesn't vapor lock in summer. Vapor lock occurs when gasoline vaporizes completely in the fuel lines before it reaches the combustion chamber. This can cause driveability problems and stalling.

It's important to note there is no consistency in gasoline. Even the same brand formulations are different in various marketing areas. In addition, the major oil companies buy gasoline from one another. Green says that Shell doesn't refine the gasoline that it sells in some markets, choosing to purchase from other refiners instead. However, he insists that the gasoline Shell purchases meets the same specifications as the fuel it refines itself.

Frequently, switching to another brand with a different additive package is all that's required to clean off deposits and prevent them from developing in the future. Knock is also often eliminated from such a switch. If that doesn't help the problem, then move up to a higher octane gasoline.

You don't necessarily get better additives by buying a better grade of gasoline. Only if an oil company scales its additive packages for different grades of gasoline do you benefit by buying higher octane fuel. Shell and Sunoco, for instance, put an identical additive package in each grade of gasoline dispensed from their pumps.

You should also be aware that gasoline changes seasonally in geographic areas that have weather shifts. This might be especially noticeable if you fill up during the "shoulder" seasons of spring and fall when going from cold to warm weather, or vice versa. For instance, refiners but less butane (a highly volatile chemical) in summer gasoline so it won't vaporize before gets to your engine. During cold weather, more butane is added to gasoline so it becomes more volatile. In a place like Minnesota during the winter, you want that gasoline that evaporates quickly.

So what's the best advice for choosing the best gas for your car? Follow your owner’s manual recommendations, stick to familiar brand-name gas and use the lowest octane gas your engine will run on without knocking (if your engine knocks, move up a grade). Also, remember to avoid adding any aftermarket "octane-boosting" additives to brand-name gas--it already has detergents added and there is no advantage to running high-octane gas through a low-octane engine.