Premium, Supreme, Ultimate. Higher-octane petrol certainly sounds much more exciting than plain old “regular”. It also comes gushing with claims. According to fuel companies, it “cleans vital engine parts” and “improves engine efficiency and overall performance”. So should we all be using premium petrol?

For most motorists, no. Using premium 95 or 98 petrol offers only marginal benefits over regular 91. The “premium” tag doesn’t mean the fuel is better quality, as all petrol sold in New Zealand has to meet stringent quality levels.

What's the difference?

The octane rating of petrol (91, 95 or 98 in New Zealand) signifies its ability to resist detonation. An engine is tuned to use petrol of a certain octane. In the engine, petrol is compressed by a piston, then ignited by a spark. The resulting explosion pushes the piston down. If the petrol detonates too early, which can happen when using a lower-octane fuel than the engine needs, it can try to force the piston down before it has reached the top of its stroke. This is noticeable as a “knocking” sound in your engine and can cause serious damage if left unchecked.

Most car engines are designed for regular 91 octane petrol. Japanese cars usually use regular 91 petrol as that fuel is widely available in Japan. But some exceptions in New Zealand need premium 95 octane petrol. These include older models and many European-designed cars (the lowest octane fuel readily available in Europe is 95). While modern cars that need premium petrol can tolerate some use of regular 91, long-term use of a lower octane could damage the engine.

What about those claims?

Using higher-octane fuel in a car that isn’t designed for it is unlikely to result in improved performance or fuel efficiency.

Some manufacturers claim their particular mix of premium additives makes a difference. For example, Z Energy says its ZX Premium 95 octane petrol (with a “friction modifier”) returned 3% better efficiency than another 95 octane fuel in its testing.

All fuel brands claim the additives in their premium petrol clean your engine, resulting in vague claims of “improved engine efficiency and performance over time” (Caltex).

However, these types of claims regularly come with disclaimers, such as “benefits are based on drive cycles in a laboratory and may not reflect real-world driving results” (BP). As they are practically impossible to verify, we’d question their value to motorists.

Pricey choice

Premium petrol comes at a price. In a review of 11 service stations (5 brands) in Wellington on 27 January, premium 95 (sold at 9 locations) was 7 to 11c per litre more than regular 91. Premium 98 petrol was 17c per litre more than regular 91 at the one service station offering it.

Most service stations don’t show their premium petrol prices on a prominent roadside board. At the 11 service stations we visited in Wellington, just 1 (Caltex) displayed its premium petrol price on a roadside board. That means you won’t know the cost of premium petrol until you drive up to the pump.

Not all garages sell all premium grades. Gull only sell 91 and 98 octane and while BP has 95 in the North Island, it has only limited availability in the South Island. So depending where you fill, you may be stung for the extra cost of 98 petrol.

Our advice

Use the lowest octane your car needs — for the majority of motorists, that’ll be regular 91. If you are unsure, check your manual. It might also have a sticker inside the fuel flap. You could also ask your car manufacturer or at a dealer for the brand.