Most gas stations offer three grades of gasoline: Regular, mid-grade, and premium. However, many consumers aren’t sure which grade of gas they should put in their car. Will premium gas really help your car perform better or keep your fuel system cleaner?
In short, the only time you should use premium fuel is if your car’s manual recommends or requires it. If your car was made to run on regular gas (87 octane), there’s no actual benefit to using premium gas.
Understanding Octane Grades
Contrary to what many people think and what the oil companies would like us to believe, higher grades of gasoline do not contain more energy for your car to run. Gasoline is rated by octane. Generally, regular is 87 octane, mid-grade is 89 octane, and premium is 91 or 93 octane. Octane ratings indicate the gasoline’s resistance to pre-ignition.
Since the ratings are an indication of resistance pre-ignition, it’s a good idea to understand how pre-ignition works. Engines work by compressing a mixture of fuel and air and igniting it with a spark. One way to get more power out of an engine is to increase the compression of the fuel-air mixture before burning it, but these higher compression ratios can cause the fuel to ignite prematurely. The premature ignition is what is referred to as pre-ignition, and is also known as knock because it makes a soft knocking sound, not unlike the gurgling of a coffeemaker.
Higher octane gasoline is more resistant to pre-ignition, which is why high-compression engines, often found in luxury or sports cars, require premium gasoline.
Decades ago, pre-ignition could cause serious and expensive internal engine damage. Modern engines have knock sensors that detect pre-ignition and recalibrate the engine on the fly to avoid it.
Pre-ignition is still bad for your engine, but it’s less likely to occur.
Using An Octane That’s Too Low or Too High
If you use too low an octane — i.e. regular gas in a car that requires premium — the engine will produce slightly less power and get lower gas mileage. Engine damage, though unlikely, is still a possibility.
If you use too high an octane — i.e. mid-grade or premium in a car that requires regular — you’re just wasting money. Many gasoline companies advertise the additives in their expensive gas; in reality, all gasoline contains detergents to help keep your fuel system clean. Some people swear their cars run better on premium gas, but the effect is largely psychological. A healthy engine designed for regular gas can’t benefit from a higher octane rating.
How to Know Your Car’s Requirements
If your owner’s manual says to use an 87 octane gasoline, you’re in luck! Think of all the money you’ll save by buying cheap gasoline. There’s no advantage to running mid-grade or premium gas in your car.
If your car has a label saying “premium fuel required,” you should always buy the higher grade fuel. Your car’s knock sensor should prevent problems, but it’s better not to risk it. Besides, running lower octane can lower your car’s fuel economy, so buying cheap gas is a false economy.
If your car says “premium fuel recommended,” you have some flexibility. You can safely run regular or mid-grade, but you’ll get better performance, and possibly better fuel economy, on premium gas. Try tracking your fuel economy on different grades of gas; fill the tank and reset the trip odometer, burn through the tank, then refill and divide the number of miles you drove by the number of gallons it took to refill. The result is your MPG, or miles-per-gallon. From there, figure out what type of gasoline gives you the best performance and economy.
Diesel Engine Options
In the U.S. and Canada, most stations feature a single grade of diesel fuel, which may be labeled ULSD, or Ultra Low Sulfer Diesel, so there are no hard choices to make. At most stations, the diesel pump is green. Do not put regular gasoline in a diesel vehicle’s fuel tank. The engine won’t run on gasoline and the repairs are expensive!
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