You might notice that when you fill up your car, you have a choice between different grades of gasoline (usually three). Most gas stations carry “regular”, “mid-grade”, and “premium” octane gasolines. In the U.S., regular is typically 87 octane, mid grade is 89, and premium is 91 or higher.
What is octane? The octane rating is a standard measure of a fuel’s performance. The higher the octane number, the more compression the fuel can withstand before igniting. Higher octane fuels are usually used in high performance or luxury vehicles that require higher compression ratios.
So, if higher octane fuel performs better, does filling your car with premium lead to better performance? It should make your car faster, keep your engine cleaner, or maybe give you better gas mileage, right? Wrong. In simple words – it’s a waste of money. Unless your owner’s manual recommends that you use higher octane gas, there is usually no benefit to putting premium into a car designed for regular, although it doesn’t hurt or damage anything. All gasolines are required to have the same amount of detergents in them, and should keep your engine just as clean.
The only case in which you might want to fill your car with premium is if you are having problems with your engine knocking. Engine knock, or pinging, occurs when a separate pocket of air-fuel mixture ignites after the spark has ignited the air-fuel mixture within the combustion chamber. Higher octane gas will help prevent this, but if your car is knocking on regular gas, you should find a more permanent solution such as retarding your ignition timing or lowering your compression ratio. Engine knock is a very serious problem, and in severe cases can lead to irreparable damage to your engine’s cylinder walls and pistons, possibly leading to you needing a whole new engine block.
However, if your owner’s manual recommends that you fill your car only with premium gas, using a lower octane even once can cause knocking and void your warranty. But not all premium gas is the same – take notice of the octane number when you fill up and make sure that it meets or exceeds the manufacturer’s recommendation. Don’t take the risk just to save a few dollars.
Another, more surreptitious difference in gasolines is summer and winter blend. Many people don’t even know or care that the gas they use in the summer is different from in the winter. However, they may notice that gas prices seem to skyrocket every spring. This is often attributed to refineries being “down for maintenance while transitioning from winter blend to summer blend gasoline”. But why is this necessary?
The difference between summer and winter blends involves a standard called the Reid Vapor Pressure (RVP) of fuel. RVP is a measure of how easily the fuel evaporates at a given temperature. Higher RVP gasolines are more volatile and evaporate at lower temperatures. Winter blend fuels have a higher RVP so that the fuel can evaporate at low temperatures for the engine to operate properly, especially when the engine is cold. If you have a car that’s been sitting with summer gas and try to drive it in the winter, it may be hard to start and run rough.
Summer blend has a lower RVP to prevent excessive evaporation in hot temperatures, which decreases emissions that contribute to the ozone and smog level, and also helps prevent problems such as vapor lock on hot days, especially on older vehicles. The EPA claims that summer-blend gasoline contains 1.7 more energy than winter-blind gas, which gives you slightly better gas mileage. However, this is usually offset by higher gas prices due to higher production costs.
The switch between fuels happens twice a year: once in the fall and once in the spring. The change requires significant work at refineries, which is why gas prices usually spike around those times. But as a driver, you should understand that there are reasons for the switch and that running the proper blend can help protect your car, even if it may hurt your wallet.