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My husband insists on using premium gas in our SUVs to pamper them, even though their manufacturers say they don’t require it. When gas prices got especially high, we couldn’t afford to go anywhere. Is there any benefit to using premium in a vehicle that doesn’t need it, or is it a waste of money? – Caroline, Vancouver

Knock, knock. If your car’s built for regular, paying a premium for premium gasoline is likely only going to benefit the oil company.

That’s because, despite the name, premium gas isn’t necessarily better. It has a higher octane rating to keep it from causing knocking in high-pressure engines.

“My brother-in-law says he puts premium in his truck to give it a treat,” said Steve Elder, automotive instructor with the British Columbia Institute of Technology’s School of Transportation. “But if your vehicle is built for 87 octane and you use a higher octane, it does absolutely nothing.”

The octane rating measures gasoline stability – it tells you how likely it is that gasoline will start to burn on its own. The higher the number, the more stable the gas is.

Regular gas has an octane rating of 87 and premium has 91. There’s a midgrade, which usually combines regular and premium, that has a rating of 89. Some places offer superpremium fuels, which go by various names, with 93 or 94 octane.

So, why does it matter? In gas-powered cars, the spark plugs ignite the gasoline vapour inside the cylinder. It’s a tiny controlled explosion. But if the gas ignites on its own, before the spark plug can do it, which can happen in the higher-temperature, higher-pressure environment of a turbocharged or supercharged engine, you’ll get what’s known as knocking. You’ll hear a knock or a ping.

It’s hard on your engine. If left unchecked, knocking can eventually damage it, Elder said.

In most cars, regular gas won’t cause knocking – but it might in turbocharged or supercharged engines.

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That’s where higher-octane gas comes in. It needs a higher temperature to ignite, so it’s less likely to cause knocking in a high-performance engine.

The price can be up to 40 cents more per litre for premium, even though it’s not necessarily better.

Last week, after gas prices had dropped slightly, a downtown Vancouver gas station was selling regular gas (87 octane) for nearly $1.96 a litre, midgrade (89 octane) for nearly $2.30 a litre and premium (91 octane) for nearly $2.38 a litre.

On a 2022 Honda Pilot with a 74-litre gas tank, for instance, it would cost about $175 to fill up from empty using premium gas.

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If you switched to regular, that fill-up would cost about $145 – saving you $30.

How do you know what kind of gas your car needs? Some auto makers put the information on a sticker next to the gas cap. Otherwise, look in your owner’s manual to see what octane rating of gas the manufacturer recommends.

There’s a difference between recommended and required. If the manual recommends midgrade or premium, you can usually use regular without any problems, Elder said.

The car’s computer knows what octane rating you’re using and adjusts the engine timing accordingly, to minimize the chance of any problems. If you end up getting knocking, switch back.

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“With all the engine monitoring being done [by your car], now, I don’t know whether [knocking or damage] would be a possibility,” Elder said. “You might notice a difference in performance, but I don’t think it would be very likely.”

In 2019, Car and Driver measured the performance of four kinds of vehicles, using regular and then premium gas, completely draining the tank between runs. In its tests of a Honda CR-V, which takes regular gas, it saw a seven-horsepower gain and a 1-per-cent improvement on fuel economy at 75 miles per hour.

But if your car actually requires premium gasoline – and unless you’re driving a high-end sports car, yours probably doesn’t – you should use it, Elder said.

What about all those detergents advertised in commercials for premium gasolines? All gas contains detergents to keep your engine clean from carbon deposits. Oil companies mostly keep their formulas, and the amounts of them in each grade of gasoline, a secret.

Even if there are extra detergents in premium grades of fuel, they’re probably not worth the extra cost, Elder said.

When to buy premium

But if you’re letting your car sit unused for six months or longer, Elder finds that a premium gasoline that doesn’t contain ethanol is less likely to go stale in the gas tank.

Gas can start to break down after about three to six months. It becomes thicker and less combustible. That can lead to rough idling and sometimes even the “check engine” light going on.

While the maximum amount of ethanol allowed in regular gasoline is 15 per cent, the average gasoline contains 6 to 7 per cent, according to Natural Resources Canada.

While the department said all gasoline can degrade over time, whether it contains ethanol, Elder finds that gasoline with ethanol degrades faster.

Several companies, including Shell and Canadian Tire, advertise ethanol-free premium gasoline.

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