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91 or 95 (or 98) octane petrol – which to use?

Petrol pump

Motorists often ask, "even though the manufacturer says use 91 octane petrol, what’s the benefit from using 95 or 98 octane instead?"

In short, there’s none; you should use what the manufacturer has specified for your vehicle.

Despite what some people claim, there is nothing to be gained from running your vehicle on a higher octane petrol than that specified by the manufacturer. Unless your engine is knocking (more about that later), buying higher octane petrol is a waste of money.

Of course, if your car is designed to run on 95 or 98 octane petrol, you must use the octane rating that has been specified by the manufacturer, otherwise you risk causing damage to the engine. If you don’t know what fuel you should be running your vehicle on, seek advice from the local dealer that services your brand of car.

Compared to 91 octane petrol, 95 and 98 octane generally cost a further 10 to 15 cents per litre; these extra prices can vary around the country depending on location and competition between retailers.

A typical 2.0 litre car using 8.0 litres of fuel every 100km and travelling the national average of 12,500 km/year uses around 1,000 litres of fuel a year. If, as specified, that car uses 91 octane petrol, the yearly petrol costs will be around $2,250. If 95 octane petrol is used, those costs rise to around $2,330 per year while using 98 octane fuel would see costs increase again to around $2,410 per year. That’s a difference of between $80 and $160 per year for no real gain in performance or fuel economy.

Because they have a higher octane rating, the 95 and 98 octane petrols contain slightly more energy and thus will provide a very small increase in fuel economy – but not enough to overcome their extra cost.

While it might seem that using higher octane petrol than specified might be ‘nice’ for your engine, or that it could ‘unleash some previously undiscovered power’ within the engine, you are not really doing anything for it at all. Most cars in New Zealand are designed and tuned to run on 91 octane petrol.

For most owners, using higher octane petrol than that specified in the owner’s handbook or listed on the inside of the fuel filler cap provides no benefit. Your car won’t go any better, it won’t be any faster or run cleaner, the fuel use might reduce fractionally but that’ll be more than offset by higher fuel costs.

About the only time you might need to switch to petrol with a higher octane level is if your car engine knocks (sometimes called pinking) when using the recommended petrol. It’s caused by the petrol and air mix inside the combustion chamber of your car igniting before it is supposed to, which you hear as a ‘pinking’ sound from your engine. These days this is not a common problem, but can cause damage to your engine: if you suspect this is occurring, take your car to an experienced mechanic for advice.