Gasoline Explained

What's the difference in gasoline grades?
What's the color of gas?
What happened to leaded gas?

Premium gas is a misnomer. Premium, plus, and regular are not value descriptions but instead are fancy terms for the octane rating. For high performance vehicles, use the right octane rating to prevent engine knocking and keep your car running smooth. For normal vehicles, plus and premium grade gasoline provide no added benefit and are not worth the extra expense.

Check your owner manual to see what gasoline grade is required in your car.


16.5 million American drivers wasted more than $2.1 billion dollars in 2015 by using premium-grade gasoline in vehicles designed to run on regular fuel. Terms like plus and premium don’t mean better quality and it doesn’t mean a cleaner engine. Don’t pay more for no benefit!


Gasoline is made up of different hydrocarbon molecules, ranging from heptane (seven carbon atoms and 16 hydrogens) to decane (10 carbons and 22 hydrogens) and others. The hydrocarbon identified on the gas pump is octane (eight carbon atoms and 18 hydrogens).

Physical contact with gasoline is serious. Contact with eyes will cause blindness and contact with ears will impair hearing. A drink of 12 ounces would be fatal to most adults.

Grades of Gasoline
An octane rating (or octane number,) is a standard measure of the performance of an engine fuel. The higher the octane number, the more compression the fuel can withstand before igniting. Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane and n-heptane. In most areas of the US, the octane rating is displayed as the standard grades: 87 (regular), 89-90 (plus) and 91-94 (premium) AKI. Despite the octane rating, all gasoline grades contain the same amount of chemical energy. In high elevation states, 85 was the minimum octane sold but with the widespread use of automotive computer controls that adjust for altitude, this low octane is falling out of favor and may someday be banned.

The required octane rating for most vehicles is octane 87. There is no benefit from using an octane higher than recommended in the owner’s manual. Some car models require higher octane because of high-compression engines and electronic controls, which are designed to create efficiency by converting a greater percentage of the gasoline’s heat energy into power.

A car's engine essentially contains a series of constant explosions. Scientific American describes the process as:

During the four-stroke cycle of a typical car motor, the piston drops in the cylinder, allowing it to fill with a mixture of gasoline and air. The piston then moves up again, compressing the fuel mix and, when it reaches the top, the spark plug ignites the explosive vapor, driving the piston down again. As the piston returns to the top of the cylinder it expels what remains of the spent fuel out through the exhaust valves and the whole process starts again. Knock occurs when the compression of fuel and air mixture alone, and not the spark plug, sets off an explosion.

Using the right octane rating will prevent knocking which is an unregulated explosion in an engine chamber. High performance engines, like in sports cars or vintage cars, will knock without premium grade gasoline. Knock is bad for engines and creates a very loud noise and vibrations. Octane gasoline resists exploding better than heptane gasoline.

Leaded Gasoline
Originally, lead (tetraethyllead) was introduced to gas to reduce knocking. Almost all countries in the world have now phased out automotive leaded fuel. When the damage to the environmental and public health caused by the lead was discovered, leaded gasoline started to be phased out (though not immediately, it took 50 years) in the US in 1973. In 1996, the U.S. Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. The elimination of leaded gasoline has increased IQ scores, lowered lead-in-blood levels by up to 90 percent and prevented the premature deaths of more than 1.2 million people annually, according to a study by Thomas Hatfield and Peter L. Tsai, of California State University.

Color of Gasoline
In some countries the law requires that low-tax fuel is dyed to deter its use in applications intended for higher-taxed ones. In the US, aviation gasoline is dyed to identify the octane rating and distinguish it from clear kerosene-based jet fuel. American gasoline is slightly yellow or mostly clear. Old gasoline turns an orange color.


Check your owners manual for the required octane level, the key term being required. Even if your manufacturer recommends premium gasoline, the car will typically run on regular without issue and won't damage the engine in any other way. The car's performance might suffer very slightly, for example lose a half-second from zero to 60 mph. Big deal!

If your car requires a higher octane level check the labels when traveling. The octane rating of gas labeled premium or regular isn’t the same everywhere. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium.



Save Money and Stop Buying Premium Gas on Edmunds
Great article about saving money with regular gas and when and if to use premium gas. There is also a list of vehicles required and recommended to use premium to help owner's make the right decision at the pump.

Gasoline on Wikipedia
Everything you need to know about gasoline.


What's Really In Our Gasoline? on Road and Track
More on what is in gasoline and specifically what's in it. The article discusses things like detergents and how it affects build up on the engine parts.

Where Does My Gasoline Come From? from the Department of Natural Resources of the State of Louisiana
Describes what exactly is in a barrel of oil, distribution, and the refinery process.