The most common grade is 91 octane, which makes up nearly 80% of all our petrol sales. Unsurprisingly, all service stations sell it. But not all cars are designed to run on the low-octane fuel, so every Caltex, Challenge and Z station, and most G.A.S, supermarket and unbranded sites sell 95 octane. So, too, do BP and Mobil, but not everywhere. At many BP and some Mobil service stations, and at all Gull sites, they offer a 98 octane grade instead of 95. In Gull’s case, the 98 grade is an E10 bioethanol-mineral blend, as it is at five Mobil sites in the greater Wellington region. Of the 280 BP stations nationwide, 99 sell 98 octane, while 31 of Mobil’s 190 sites sell 98 octane mineral fuel (in the North Island only).

The ‘premium’ (or ‘super’) moniker associated with high-octane fuels suggests a superior product that might translate to better performance or value for money but, in reality, all fuels retailed in New Zealand must meet a similar specification which is amongst the most stringent in the world. The Consumer Affairs unit of the Ministry of Business, Innovation and Employment randomly tests fuels to confirm they meet that standard. High-octane mineral fuels do have marginally higher energy levels than lower grades, so fuel economy may improve slightly. They also typically contain more additives designed to reduce carbon deposits and burn cleaner, although the type and amount of additives in the fuel varies between fuel companies. 

Higher octane fuels also cost more. Today, the differential between each mineral grade is eight cents per litre (less for bioethanol blends). That means 98 octane typically costs 16 cents per litre more than 91 octane, and eight cents more than 95. Not so long ago, the differential was only four cents per litre, but rising production costs for high-octane fuels have widened that gap. Unfortunately for motorists, it’s often unclear which ‘premium’ fuel (95 or 98 octane) is sold at which service station, as retailers are not required to display the price on the price board – only at the pump. On occasion, that means some motorists may unintentionally spend an extra eight cents per litre buying a higher octane fuel than the one they actually need.

With motorists keen to manage rising fuel costs, the AA wanted to understand whether the higher octane fuels are worth the extra money when used in a car configured to run on a lower grade. 

So, why have different fuels? The octane rating relates to the engines’ compression ratio, which determines the amount of compression the fuel can handle before it ignites. The higher the octane number, the more compression the fuel can withstand before detonating. Broadly speaking, fuels with a higher octane rating are used in high-compression engines which have higher performance. 

In Europe, 95 and 98 octane are the two most common grades; in Japan it’s 91 and 96/98 octane. In the USA, the fuel grades range between 87-91octane. Australia mostly uses 91 or 95 octane.

A 2005 assessment estimated that 95% of New Zealand’s cars could operate on 95 octane or less, while 4% of vehicles may benefit from a higher octane fuel, but less than 1% actually require a fuel grade above 95 octane. Few modern vehicles need 98 octane; a small number of older or classic cars do, although many of those can operate happily on 95 octane, with an engine detune.   

To see if there was any difference in economy, we ran a car designed to use 91 octane on 15 litres of that grade to measure its fuel consumption and then, when the car had completely run out of fuel and wouldn’t re-start, we refilled it with 15 litres of 95 octane and repeated the route until it too ran out. Our expectation was that, as higher octane fuels contain marginally more energy, there would be a slight improvement. As this was an economy test only, we did not measure performance improvements that motorists may experience with higher octane fuels.  

On the day of the test, our test car, a 2010 Suzuki Swift 1.5, travelled 247km on the 91 octane petrol. That’s the equivalent of 6.07 litres per 100km, or 16.46km per litre (see table). On the 95 octane blend, the same car on the same day travelled 256km, or nine kilometres more, for an average economy of 5.86 litres per 100km (3.5% less), or just over 17km per litre. 

That’s an improvement on the higher octane fuel, but it costs eight cents per litre more, so how do the costs really compare over a year? For a typical Kiwi motorist travelling 14,000km per year, we estimate the Suzuki driver would consume 850 litres of 91 octane at a total annual cost of $1699, assuming a fixed price of $2.00 a litre for ease of comparison. 

If the same driver only used 95 octane, they’d consume 820 litres, at a total cost of $1705 a year. While that’s 30 litres less, the cost is $6.86 more. Although negligible, our test results suggest motorists would receive little or no financial gain by using 95 octane in a car configured to run on 91, despite the improvement in economy. 

The next test ran a 2011 Subaru Impreza WRX, designed to use a minimum 95 octane, on 15 litres of that grade until it too ran out, and then the exercise was repeated on 98 octane. On the 95 octane the Subaru travelled 170km, for a fuel economy rating of 8.82 litres per 100km, or 11.33km per litre. On 98 octane it travelled 171km, for a 0.57% improvement in economy.

Over 14,000km the Subaru driver would consume 1235 litres of 95 octane petrol at a total cost of $2567 a year, assuming a price of $2.08 a litre. On the pricier 98 octane, they’d consume 1228 litres – just seven litres less – at a total annual cost of $2651, or $84 more. These results suggest that the owner of a car optimized to run on 95 octane would receive no financial benefit from using 98 octane.

Because a vehicle’s octane rating is not always known, the AA is asking Consumer Affairs to amend the Motor Vehicle Sales Act to require octane ratings to be published on the vehicle’s Consumer Information Notice. This data could also be printed on a sticker inside a vehicle’s fuel filler flap.

Presently, dealers are required to inform prospective buyers of the radio receiver capability, since domestic Japanese car stereos are not compatible with New Zealand’s FM bandwidth. Given the low price of car stereos these days, the potential cost to Kiwi motorists of using the wrong premium fuel (when some might not need premium at all) could be far more than the cost of a car stereo during their ownership – and, collectively, millions of dollars. The AA thinks if it’s considered important enough to record car stereo capability, then providing octane ratings should be mandatory.

As for motorists identifying which grade of petrol they are buying, the AA wants service stations to display the price of all fuels they sell on the price board. We say their main reason for being in business is to sell fuel, so the price of those core products should be clearly displayed to help motorists choose and to encourage competition between sites. The AA will also lobby to regulate fuel price boards.

In the meantime, the AA advises motorists to check the grade and price at the pump. If the premium fuel costs over eight cents per litre more than 91 octane, it signifies 98 octane fuel. 

As for that price differential, the AA says eight cents per litre is too much. Our monitoring of commodity prices suggests 95 octane should retail for about five cents per litre more than 91 octane.  

Which fuel?

If you’re not sure of your vehicle’s correct octane, check the owner’s manual or ask the manufacturer or dealer. It’s harder to verify for used cars, especially imports, so it’s not uncommon for used car dealers to advise buyers to use 95 octane as a default option, even though most Japanese cars are designed to use 

91 octane. That’s good advice, because using a lower grade petrol than that recommended by the manufacturer risks long-term damage to the engine of older cars, whereas there is no harm in using a higher grade than specified. 

However, advanced fuel and ignition management systems in 

late-model vehicles will compensate for a lower octane fuel to prevent engine damage.

Irrespective of price, the AA strongly advises against using a lower octane fuel than that recommended by the vehicle manufacturer and, if in doubt, to use a higher grade.

  2010 Suzuki Swift GLX  2011 Subaru Impreza WRX
  91 95 difference (91-95) 95 98 difference (95-98)
Price/litre  $1.999  $2.079 $-$0.08 $2.079 $2.159 -$0.08
x 15 lites  $29.985  $31.185  -$1.30 $31.185  $32.385  -$1.20
Distance travelled (km)  247 256 -9 170 171 -1
Km per litre 16.46 17.06 -0.6 11.33 11.4 -0.07
Litres per annum (14,000km)  849.8 820.4 29.4 1234.8 1227.8 0.05
Litres  849.8 820.4 29.4 1234.8 1227.8 7
Cost per annum $1,698.75 $1,705.61 -$6.86 $2,567.15 $2,657.15  -$83.67

Reported by Mark Stockdale for our AA Directions Summer 2018 issue

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