Most vehicles are pretty specific about what type of fuel you can use: it’s either regular-grade or “premium required”. But every now and again, you’ll come across one that says “Premium Recommended”. And that might have you wondering why there’s an option, and what fuel you should give it.
The short answer is that the engine’s sensors are set up so it can use either regular or premium – and your choice depends on how much power you want your engine to deliver. You’ll get a small bump in horsepower on premium fuel, but of course you’ll pay more at the pump.
The numbers on the gas pump indicate how much octane the fuel contains: 87-octane gas is considered regular-grade; 89-octane is mid-grade; and 91-octane and above is premium. Octane is a hydrocarbon molecule, but it isn’t an additive that’s mixed into the fuel. Rather, the amount depends primarily on how the fuel was refined.
Higher-octane fuel is less volatile than regular-grade, and that makes all the difference in how it burns under specific conditions in the engine. With any fuel grade, when the spark plug fires, the air–fuel mixture in the combustion chamber doesn’t instantly burn all at once. Instead, the spark ignites the fuel molecules closest to it, creating a “flame front” that spreads throughout the rest of the fuel.
Ideally, the flame front should advance gradually (of course, everything’s relative: we’re talking milliseconds here) to ignite everything smoothly and as completely as possible, with a minimum of wasted unburned fuel.
However, as the piston compresses the air–fuel mixture prior to the spark plug firing, it raises the temperature. This can potentially cause improper ignition, with various points of combustion igniting throughout the chamber. This creates shock waves, which you hear as a pinging or knocking sound. In extreme cases, this heat can cause the fuel to spontaneously combust before the spark plug fires, a condition known as pre-ignition.
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In an engine labelled for 87-octane, regular-grade fuel will usually create a stable flame front. But higher-performance engines have higher compression ratios, which create more heat as the fuel and air are squeezed. This is why most of them require higher-octane fuel: that lower volatility means it’s far more willing than regular-grade to wait for the spark plug to fire.
Pre-ignition can seriously damage an engine, and in days gone by, drivers who’d repeatedly filled their tanks with the wrong fuel were often forced to replace their engines afterward. But with engine computerization came knock sensors. When these sensors identify an issue, the ignition system adjusts the timing and fuel delivery so the spark plug fires at the appropriate time for the mixture.
In a high-performance car, this can make a considerable difference in power and fuel economy. In extreme cases, the car might just limp along, but the engine won’t destroy itself.
But in vehicles with a “Premium Recommended” guideline – which will be written on the fuel cap or fuel door, and in the owner’s manual – this adjustment is minor. Conservative drivers might not notice any difference, while those with a heavy right foot might feel a slight drop in hard acceleration.
That variable-octane recommendation gives you the option of choosing lower pump prices or higher performance. You’ll get slightly worse fuel economy on regular grade, but that fuel’s lower price will likely make up the difference. It’s safe to switch grades at any time; you don’t have to pick one and then stick exclusively to it.
On any vehicle, choosing your fuel can depend on several factors. Here’s our advice...
- If you hear your engine pinging or rattling, or if you’re not getting the power you usually do, try moving up to the next grade. Engines rated for 87-octane can become susceptible to these issues as they get older. This may be more evident in extreme conditions, such as when you’re towing or if it’s very hot outside.
- If your engine is rated “Premium Required”, we recommend you use it. We’ve read all the online stories about how it isn’t really necessary, but consider it cheap insurance to use the right fuel – especially if, down the road, your engine needs repair and your warranty won’t cover it because you used lower-grade gas.
- Some drivers like to give their 87-octane engines an occasional tank of premium, but there’s no need to move up if you haven’t experienced any issues. By law, regular-grade fuel contains minimum-standard levels of detergents and additives. If you don’t think that’s enough, many major retailers sell Top Tier gas, a privately managed fuel program that adds higher amounts than the minimum standard. All Top Tier octane grades have the same additive levels.
- Follow the maintenance schedule in your owner’s manual, including servicing or replacing the spark plugs when recommended. They last a long time, but when a plug finally wears out or gets too dirty it won’t fire properly, and will affect your engine’s performance.
- On some vehicles, especially with European brands, you might see two sets of fuel recommendation numbers, and the one marked RON will be higher. Octane concentration is measured by Research Octane Number (RON) and Motor Octane Number (MON). Europe generally rates its fuel at the pump by the RON alone. In Canada and the US, the RON and MON are averaged out, and the resulting octane numbers – 87 and up – are called AKI, for Anti-Knock Index. That’s the number to match at the pump.