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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. 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. 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. 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. 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.
Use diesel fuel if your vehicle is designed for it. Understand what an octane rating means. Don't assume higher octane improves performance. Check in your vehicle's owner's manual. Check the automaker's website.