When it comes to the fuel they prefer to drink, most vehicles are pretty clear. Either regular-grade is okay, or you need to pump high-test into the tank.

But some stand in the middle, at “Premium Recommended.” So what do you use, especially when the difference may be a dime or more per litre?

The answer depends on whether you prioritize cost or performance, which is why it’s important to understand how an engine can accept more than one grade of fuel.

Let’s start with the basics. When the gasoline (mixed with air to form a vapour) enters the combustion chamber, it’s ignited by the spark plug. The resulting energy moves a piston to spin a central crankshaft. That shaft’s spinning motion eventually turns the wheels.

Before it’s ignited, the gas/air mixture is compressed by the piston, which creates heat. If it gets hot enough, the fuel can spontaneously ignite before the spark plug fires, a condition known as pre-ignition. It’s also called engine knock, because of the sound it makes.

In older cars, pre-ignition can cause serious engine damage. Newer cars have knock sensors that will adjust the timing to ensure that the spark plug ignites the fuel at the right time.

When the system sets back the timing, though, you’ll get less power from the engine. If it’s minor, you might not notice any difference, but in extremely severe circumstances, the car will be very sluggish.

Now…about octane. It sounds like an additive, but it’s actually a molecule whose concentration in the fuel is determined by how the gasoline is refined. Processing a high-octane stream is more complicated, so the fuel is more expensive.

The octane concentration is measured by the Research Octane Number (RON) and Motor Octane Number (MON). Europe primarily uses the RON alone, while Canada and the U.S. average the two numbers to obtain the Anti-Knock Index (AKI).

That’s the number you see on the pump: 87 for regular, 89 for mid-grade, and 91 for premium. Some stations also carry a higher-grade 94 octane fuel.

With those flashy high numbers, you’d think that octane’s a fierce fuel. In reality, it’s less volatile, and so higher-grade gas is less likely to spontaneously ignite. That’s why it’s generally required in performance-oriented vehicles, which usually have higher-compression engines where pre-ignition is more likely to be a problem.

If 87-octane is what your owner’s manual says, then there’s no need to buy anything higher. You’ll just be wasting your money.

But if your vehicle is “premium recommended,” then you can choose your grade. You’ll have a reduction in horsepower at the lower grade, but it won’t harm your engine.

Our suggestion? Try a tank of the lower-grade fuel and see how your vehicle works. You may be fine with its performance, and can save yourself a few bucks when filling up.

But if it seems sluggish, or if you hear pinging or rattling in the engine when you accelerate, move up a grade. If it still doesn’t feel right on mid-grade, you’ll have to pop for the expensive stuff.