How to Choose the Right Fuel Type for Your Car

When to use regular, mid-grade or premium gas

Close Up of a Gas Pump
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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.

Here's how pre-ignition works. Engines compress a mixture of fuel and air and ignite them 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, hence the term pre-ignition. It's also known as "knock" because it makes a soft knocking sound.

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

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.

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.

Using Premium Fuel in Older Cars

If your car is really old — we're talking 1970s or earlier — you may need to use 89 octane or better, and you should listen for pre-ignition knock. If you hear it, it probably means your car needs a tune-up, not better gas. If your car was made since the late 1980s, use whatever fuel is recommended in the owner's manual. If the car runs poorly, that could be a sign that the fuel or ignition system needs cleaning or adjustment. It's best to spend money on having the engine tuned up rather than buying more expensive gas.

High Altitudes and Lower Octane Gas

If you're driving in the mountains, you'll often find gas stations with lower-octane gasoline, for example, "85 octane regular" instead of "87 octane regular." This is because the air density is lower at high altitudes, which affects how the fuel burns in the engine.

Choose your gas according to how long you'll be staying. If you're spending the week, it's safe to tank up on the fuel that corresponds to what you normally use, such as regular or premium. If you're just passing through, plan for lower altitudes and go by the numbers on the pump. If your car requires 87, then use 87 or higher. If your car requires premium, buy just enough gasoline to get you back down to lower altitudes, then tank up on 91 or 93 octane once you've reached your typical altitude.

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 Sulfur 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.

Biodiesel Fuel

Some stations offer biodiesel blends denoted by a BD label, such as BD5 or BD20. Biodiesel is made from vegetable oil, and the number indicates the percentage; BD20 contains 20 percent biodiesel and 80 percent petroleum-based diesel.

Check your owner's manual to see if your engine is BD-capable, and if so, to what percentage. Most new cars are limited to BD5. Biodiesel contains methanol, which can damage soft rubber components in the car's fuel system, and may be too thick to flow through the finer orifices of modern fuel injectors.

If you're interested in cleaner running, you may be able to convert your diesel vehicle to run 100 percent biodiesel or even raw vegetable oil. You can learn more about biodiesel here.

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