Choose the Right Fuel for Your Car

Choosing the right fuel for your vehicle and driving conditions is important to keeping your engine running smoothly and maximizing its longevity. If your vehicle runs on gasoline, you’ll have to determine the proper octane level for your application. The way your drive, the environment you drive in and the original design of the engine all have an effect on the optimal octane rating for your vehicle.

Steps

Educating Yourself On Your Options

1. Use diesel fuel if your vehicle is designed for it. Engines designed to run on diesel fuel operate differently than those designed to run on regular gasoline. Diesel engines do not use spark plugs, and instead rely on the compression of the air and fuel to ignite the mixture alone. Because of this, diesel fuel is less stable and cannot be used in engines designed for gasoline. Diesel vehicles usually have clear “diesel only” markings on the inside of their fuel doors, and traditional gas pumps won’t fit in their filler necks.
   • Diesel fuel usually comes in only one grade, so there are no options to choose from.
   • If you aren’t sure if your vehicle operates on diesel fuel, refer to the owner’s manual or manufacturer’s website.
2. Understand what an octane rating means. Every time you pull up to the pump, you’re presented with a few different options for the octane rating in the gas you put into your engine. Most places offer 87, 89, and 91 octane fuel, while some locations may offer higher octane options at the same or separate pumps. This rating is a measure of the stability of the fuel under compression. Higher octane fuels are more stable and less likely to pre-detonate before the spark plug ignites the air/fuel mixture. Pre-detonation (sometimes called “pinging”) can cause serious damage to your engine.
   • Engines with higher compression ratios require a higher octane fuel to ensure their air/fuel mixture doesn’t pre-detonate.
   • Most car makers design their engines with a specific octane fuel in mind.
3. Don’t assume higher octane improves performance. Higher octane doesn’t necessarily mean better performance. High performance engines often require a higher octane fuel because they utilize much higher compression ratios than most vehicles. The compression ratio is the comparison between the volume of air and fuel that can fit into a cylinder with the piston in the down position and the same volume when the piston is in the up position (or when the air and fuel are compressed). Lower compression engines do not require a high octane fuel and often won’t benefit from its use.
   • You can get the best performance, mileage and emissions under normal driving conditions from using the octane fuel your engine was designed for, not the highest octane available.
   • The octane rating an engine requires has much more to do with its design than it does with its performance.
4. Check in your vehicle’s owner’s manual. Every engine is designed with a specific octane rating in mind, so the easiest way to know what kind of fuel to use in your car is to refer to the factory owner’s manual. Many economy cars are designed to run best using 87 octane fuel because the automakers understand that a person shopping for an economy car would likely also prefer to use less expensive fuel, while many higher end cars may specifically state that you should use 91 octane. Make sure to use the octane rating specified for your vehicle in order to keep your engine running properly.
   • Some cars will state the required octane rating on your dash board near the fuel gauge.
   • Cars that require 91 octane often have a reminder printed on the inside of the fuel cap to ensure you use the proper rated fuel.
5. Check the automaker’s website. If you purchased your vehicle used without an owner’s manual or you’ve misplaced it, you can often find the required octane rating for your vehicle on the automaker’s website. If you are having difficulty locating it, try a simple search query such as, “what is the correct octane fuel for a 1998 Volkswagen Jetta?”
   • Only trust information you find on reputable websites such as the official site for the automaker.
   • Do not trust information about fuel types you should use that you find on blogs or in forums.

Knowing When to Switch to a Higher Octane

1. Use high octane on forced induction applications. If your vehicle has been modified to add a turbocharger or supercharger to increase power output, it has the same effect as dramatically increasing the compression ratio of the cylinders in your engine. These engines require a higher octane fuel due to the increased likelihood of pre-detonation. If you aren’t certain if your vehicle came with a form of forced induction from the factory, check the auto manufacturer’s website.
   • If your vehicle came turbocharged or supercharged from the factory, it likely requires premium fuel, which is considered to be 91 octane or higher.
   • If you added a form of forced induction to the engine, it also will require high octane fuel. Some aftermarket forced induction applications even require higher octane than is offered at most gas stations.
2. Switch octane ratings with older models, if necessary. While most vehicles are designed to run on 87 octane, carbon build up in the engine over years of use may result in an increased likelihood of pre-detonation. As the engine burns fuel, carbon begins to develop inside the cylinder. Over time, this can create hot spots that may ignite the air and fuel mixture early. Switching to a higher octane fuel at around the hundred thousand mile mark may help you to avoid this.
   • If your car was designed for 87 octane, you may want to switch to 89 at around the hundred thousand mile mark.
   • Not all cars will suffer pre-detonation as a result of carbon build up, but switching to a higher octane can reduce the risk if yours does.
3. Run higher octane fuel in hot temperatures. Because the octane level of your fuel helps to determine its stability under high heat and high pressure, an increase in the ambient temperature of the air can have an effect on the way your vehicle runs. Hotter air entering the cylinder has an increased likelihood of pre-detonation as it is mixed with fuel and compressed. Locations that suffer extreme highs like the desert, or any area that routinely exceeds one hundred degrees Fahrenheit may require a higher octane fuel, particularly in older vehicles with carbon buildup.
   • Exceptionally low humidity can increase the risk of pre-detonation further, so deserts may require a higher octane fuel.
   • If you will be driving through extremely high temperatures and low levels of humidity, you may want to add a higher octane fuel to your vehicle.
4. Use a higher octane when towing something with your vehicle. Research has shown that under rigorous driving, a higher octane fuel may help reduce the stress on your engine. Towing something behind your vehicle or carrying heavy loads forces the engine to work much harder. This type of driving is referred to as “severe duty operation.” Severe duty operation can be made even worse when driving in high temperature, low humidity areas like the desert.
   • During severe duty operation, running a higher octane fuel can improve gas mileage and emissions.
   • These benefits are dramatically reduced during normal vehicle operation.

Getting the Most Out of the Fuel You Choose

1. Take your budget into account. Some vehicle’s owner’s manuals will suggest that you can use a range of octane levels when choosing fuel. Automakers suggest that in vehicles designed for multiple octane ratings, using the lower octane option may compromise the performance of your engine slightly. However, research has shown that the difference in the performance of these engines while using the lower octane options has been negligible at best. For most driving applications, using the less expensive, lower octane fuel will come with no adverse effects.
   • If your owner’s manual allows for a range of octane levels, choose the one that best fits your budget.
   • Remember that the drop in the performance of your engine at a lower octane is negligible under most driving conditions.
2. Take the altitude into account. You may have heard the expression, “the air is thinner” at high altitude. The “thinning” of the air is caused by a reduced amount of oxygen in the ambient air around you. The higher you are, the less oxygen can reach you and your engine. Effectively, driving at a high altitude affects your engine in the same way reducing the compression ratio would. With less oxygen entering each cylinder, the likelihood of pre-detonation is reduced. At many locations that are 5,000 feet above sea level or higher, 85 octane fuel is sold for this very reason.
   • If you live in a high elevation area (4,000 feet above sea level or higher) you can safely run a lower octane fuel than you might closer to sea level.
   • Remember that running a lower octane fuel can become risky if you drive your vehicle to a lower elevation.
3. Add octane boosters to increase fuel stability. If you find yourself in a situation where your engine could benefit from an increase in the octane of the fuel its running, either because of extreme weather or severe duty driving, you can add ethanol to the fuel to increase its stability. Ethanol is a form of alcohol with an average octane rating of about 109, and mixing it with fuel can increase the octane rating of an entire tank.
   • Most gasoline purchased in the United States is already about ten percent ethanol.
   • You can purchase ethanol octane boosters at most auto parts stores and many large retailers.
   • Always add ethanol to a full tank of gas.
4. Look for top tier gas. Regardless of the octane level you choose, running top tier gas in your vehicle will increase the engine’s longevity, as well as possibly improving your gas mileage and performance. To be considered a top tier fuel distributor, your gas must contain at least three times the required detergent levels set by the EPA. Premium fuel often contains higher levels of these detergents, so you may opt to purchase premium fuel strictly because it will reduce the likelihood of your fuel system clogging. You can find top tier gas at a number of gas stations, including:
   • BP (British Petroleum)
   • Texaco
   • Costco
   • Exxon

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