When stopping for gas, you have probably seen some variation of the same three gas types at every gas station in the United States: Regular, Plus, and Premium. Some stations even offer up to five variations of different gas types. But what do these numbers mean on the pump? And how do you know which gasoline type is best for your vehicle? To answer this question, it’s important to know a little about how gas powers your vehicle.
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In order to understand your fuel options and which gasoline type is right for your vehicle, it helps to first understand how gasoline is used in your engine. Interestingly enough, the way gas is used in your vehicle is something similar to miniature explosions.1 These “explosions” (also known as combustion) detonate a fuel and air mixture at just the right time when the pistons in your engine move up and down. The force of the combustion is transferred to the crankshaft to propel your vehicle forward.2
The octane rating, which is the number you see on the gasoline pump, is the measure of a fuel's ability to resist "knocking" or "pinging" during combustion. This typically sounds like a clunking while you are driving. The "knocking" is caused by the air and fuel mixture detonating prematurely in the engine. The reason you might hear “knocking” is if you are using the wrong fuel, which causes the gasoline to combust at the wrong time.1
Unleaded gasoline typically has octane ratings of 87 (Regular), 88–90 (Midgrade), and 91–94 (Premium). In some high elevation areas, you may find gasoline with an octane rating of 85 due to there being less oxygen in the air, which can affect how things burn.2 Ethanol has a much higher octane rating of about 109, which is higher than gasoline.3 Ethanol can therefore provide increased power and performance if your car is made for it.4
The vast majority of automobiles require Regular (87) gasoline, but Premium gasoline is crucial for engines with a higher compression ratio. While the lower rating of 85 is available in some places, this option is only suitable for older vehicles with a carbureted engine (a tube that allows air and fuel into the engine through valves, it makes sure that the fuel-air mixture is correct), or if you are living at a high altitude, like in the state of Wyoming.5,6 Before selecting which gas type to use, consult your owner’s manual for the vehicle manufacturer's octane requirements.
You should never put gasoline in your diesel engine, but if you realize that you have, turn off your car immediately and call a mechanic to aid you in fully draining your car’s fuel lines. Similarly, even a little bit of gas in a diesel engine can destroy it. In fact, putting higher octane fuel in your car than it requires will not help it perform at a higher level. The Federal Trade Commission (FTC) says there are no advantages to using Premium gas in cars that do not require it.4 “It won't make your car perform better, go faster, get better mileage, or run cleaner,” the FTC states on their website. “Your best bet: listen to your owner's manual.” But, if you put lower octane gasoline in a vehicle that requires high octane gas, you can cause damage to the engine. Once you’ve realized that you have filled your tank with the wrong octane level, you should switch to the correct level as soon as possible. If you’ve been running on a lower octane than is required for multiple gas tank fills, you should contact your mechanic to check for damage.7
So, to best take care of your vehicle, make sure you are providing it with the correct gasoline type per your owner’s manual. Most vehicles require Regular gas and using higher octane level gas will do nothing to help, and may actually hurt, the engine in your vehicle.
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