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How Do I....Choose the Correct Fuel Grade?

Choosing the correct grade of fuel for your car or light truck is a simple as consulting the owner’s manual, or even reading the labelling on your fuel filler door. Or is it?
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The reality is that in some cases you may be able to use a less expensive grade of fuel in a vehicle that calls for premium.

If your manual or fuel door label says “premium fuel recommended”, then yes, absolutely you can use regular fuel. However, you may experience a small reduction in performance or even economy. As an example, 2.5 litre turbo versions of Mazda’s CX-5 crossover are rated 250 horsepower on 93 octane premium fuel. The horsepower drops to 227 horsepower on 87 octane regular grade, which Mazda considers completely acceptable for full-time use in that model.

When the automaker’s literature or the gas-door sticker says “premium fuel required,” greater caution is called for. A gasoline blend’s “octane rating” is actually a laboratory derived measurement of its resistance to an uncontrolled spontaneous combustion of the fuel/air mixture inside the cylinder, called “knock.”

Modern engine controls can detect and attempt to mitigate knock – which in severe cases can actually break components like pistons, connecting rods, and even valves – but they’re not infallible. Using fuel that is less susceptible to knock is the first line of defence against it.

If you have a turbocharged or supercharged engine — often found in premium or performance models — the conditions inside the combustion chambers are more challenging than those in less stressed mainstream vehicles. In these instances, you should probably stick with the automaker’s recommendation.

If your car has a premium requirement listed, but you’re not too concerned with maximizing performance, the safest strategy would be to add regular fuel a ¼ tank at a time and observe for any changes in operation, or the presence of a metallic rapping noise while accelerating. If no concerns arise, add another ¼ tank and keep an eye (and ear) out again. If all’s still well, you should be fine to continue with the lower grade fuel, though keeping in mind that extended hard acceleration should be avoided if possible.

Bear in mind that if the vehicle is still under its powertrain warranty, you could void it if something were to go wrong and it could be proven that you didn’t stick to the manufacturer’s requirements. In this case the cost benefit is probably not worth it.

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