Wondering which gas to use in your car? The vehicle’s manufacturer makes it easy to decide what to put in your tank. Just look for the fueling info in the car’s manual, or if you don’t want to thumb through all those pages, check inside your gas-tank door. A label there should indicate whether your car needs premium unleaded fuel.
Using lower-grade gas than what is required for your car may affect its power, torque, and overall performance. Your fuel economy may decrease as well. But here’s an important question: Does the language in your car’s manual suggest premium gas, or does it say your car requires it? If it’s a suggestion rather than a necessity, your car (and your wallet) will likely be fine with regular unleaded fuel.
Premium vs. Regular
Unleaded gas is considered “premium” or “regular” depending on its octane level. This determination can vary state to state, but typically, gas with an octane level above 91 is considered premium. An octane level of 87 is considered regular.
Mid-grade gas (octane level 89) splits the difference between regular and premium. But the added cost for this gas isn’t necessary unless your vehicle manufacturer requires this octane level. You can easily spot these levels at the pump based on the numbers on the buttons used to select your gas.
Gas with higher octane levels does a better job preventing unnecessary combustion in your engine. In other words, it helps ensure your fuel burns evenly, which can keep your engine cleaner.
What octane you need will be stated in your car's manual. You’ll see instructions like, “Use unleaded regular gasoline with an octane rating of at least 87 to 91,” or “at least 91 to 95.” In the former, premium could be seen as a recommendation (since the car is fine with “at least 87”), whereas it’s a requirement for the latter.
For vehicles with high-compression engines or turbochargers (think sports cars), gas with high octane levels can help maintain the health of the engine. For regular vehicles—likely those for which premium gas is recommended, but not required—studies have shown that using regular unleaded gasoline does not adversely affect your engine’s performance or cleanliness.
Your Local Gas Station vs. Name-Brand Gas
There are more than 100,000 gas stations across the nation. Some of these are name-brand retailers (think Mobil or Sunoco). Others are generic or unbranded dealers (think that mini-mart by your house that doubles as a burrito joint). Despite the gulf between those guys and, well, Gulf, their gasoline isn’t that different.
That’s because all filling stations obtain their gas the same way. Raw fuel arrives at a regional distribution center, and retailers purchase it there. Before selling it to the consumer, the gas stations blend in their own combination of detergent and/or additives to help your car run clean.
The EPA regulates the minimum amounts of these via the Clean Air Act. Those blends can differ from station to station, and some are licensed to sell fuel with the “Top Tier” additive package—which a AAA study showed is better for your vehicle. So if you opt to pay more at a brand-name station, make sure you’re actually getting a higher quality of fuel in return.
The Difference Between Winter and Summer Gas
Saving money on gas when you can is important, but sometimes the choice won’t be yours. This is the case when filling stations switch from winter gas to more-expensive summer gas each June.
Like other items related to your car—snow tires, for instance—gas is formulated differently to ensure optimal performance during cold and warm seasons. Specifically, this involves the Reid Vapor Pressure (RVP) within the fuel. RVP determines how easily fuel evaporates. In the low-temperature winter months, gas has to be able to evaporate easily, so a higher RVP is needed.
Unfortunately, the evaporation of gas isn’t good for the environment. So, when the weather turns warmer, fueling stations must switch to summer gas, which features a lower RVP—and is more expensive to produce as a result. From June 1 through September 15, gas stations cannot sell fuel with an RVP above certain thresholds.
What About Diesel?
Diesel cars require fuel specifically formulated for their engines. This is because diesel engines utilize fuel differently than gasoline engines, injecting it only after air is compressed (in a gasoline engine, air is mixed with the fuel before compression) and forcing it to combust via pressure, rather than using spark plugs.
As a result, if you opt for a diesel vehicle, you’ll need to buy diesel fuel only. On average, doing this will cost you less than premium gas, but more than regular unleaded fuel. How much you’ll pay for fuel is important to consider when creating a car-buying budget.
The Bottom Line
Some cars require premium fuel, and in those cases, you’re going to have to make sure you consistently use the fuel specified for your vehicle. But if your manufacturer only recommends a more expensive type of gas, trust your experience with the vehicle. If you don’t have any issues with regular unleaded fuel—or gas from a generic station, for that matter—you should feel comfortable using both to keep your tank and wallet full. And if you've got a diesel, things are very straightforward: You've got to use diesel fuel, every time.