Which petrol should you use?

Which Type of Petrol Should You Use?

The colour nozzle that best matches your car’s needs isn’t as simple as petrol or diesel. Canstar Blue explains.

What’s the difference between the candy-coloured pumps on the garage forecourt, and is 98 really better for your car than 91? Canstar Blue looks at the different fuel types, and which type of petrol you should use for your car.

Which petrol should you use?

The numbers on your petrol are what matter. So while petrol stations like to throw their own names and fancy terms on their fuel, you only need to focus on the digits.

Generally, your choices boil down to 91, 95, 98 and e10.

Standard unleaded petrol is 91. And this is what most of our cars happily drink. Premium unleaded is both 95 and 98. The ethanol-blended e10 (a mixture of up to 10% ethanol in petrol) is a substitute for 91 in most cars. Some distributors have other blends, such as e3 (3% ethanol).

If my car takes 91, can I use other fuels?

The numbers 91, 95 and 98 correspond to the octane rating of a fuel. This is a measure of how much compression the fuel can withstand before ignition. The higher the number, the higher the compression.

High-performance engines that use higher compression are able to extract more power out of a high-octane fuel. Although this is due to the engine’s design, rather than the actual energy within the fuel itself.

Car makers produce engines for use with fuel of a minimum octane rating. So you shouldn’t use a fuel with a lower rating than recommended. For example, putting 91 in a car that takes 95. But the reverse should be fine, such as using 95 in a car built for 91.

If you’re unsure of your car’s minimum octane rating, it should say on the inside of the fuel flap. If it says “unleaded petrol only” it means 91 octane fuel is OK. If the fuel flap says “premium unleaded only”, it means you need to use at least 95. If the fuel flap tells you to use 98, that’s what you need to do.

If you use a lower octane fuel than recommended for your car, it can affect the ignition process and cause noisy pinking or pinging, which can mechanically damage your engine.

Are there any benefits to premium fuel and will it save me money?

As mentioned above, if your car requires premium fuel, you better use it. Otherwise, you could cause some serious damage to your car. But if your car doesn’t require a high-octane fuel, is it still an option worth considering?

Fuel retailers love to talk up the purported benefits of their premium fuels. They don’t lie on this, but they do sometimes overstate the benefits. Most modern engines will adapt up (very slightly) if you run them on a higher octane fuel than the minimum recommended. You will get either better economy or more performance (depending on how you drive). But, in practice, the improvement is tiny, and the price premium of the higher octane fuel always eclipses the economic benefit from using it.

In other words, it’s not an economically rational choice to run 98 in an engine designed for 91, even though it might run slightly better. Any added mileage or improved performance isn’t significant enough to outweigh the added cost.

What about e10?

Ethanol is interesting; e10 is not a substitute for premium unleaded petrol. If your car requires 95 or 98, e10 is not a viable fuel for it. The majority of cars on New Zealand roads designed for 91 petrol can accept e10 – but you should check the owner’s manual or ask the manufacturer or dealer first.

Ethanol is an octane booster for petrol, so e10 generally has an octane rating higher than 91. (Notionally it’s 94, but in practice it depends on exactly how much ethanol there is in the “up to 10%” blend at any particular re-fill.) This means your car will perform slightly better on e10, but there is a trade-off.

The trade-off is that there is about 30% less energy in ethanol, compared to petrol. When it’s blended 10% with petrol, there is about 3% less energy in the blend. And that means your fuel consumption will increase by about 3% when you run e10, compared to 91 octane unleaded, if all other things remain equal. So that means for e10 to be an economically rational choice for you, it really needs to be about 3% cheaper than 91 octane regular unleaded.

What about diesel?

95 or 98 is more expensive, and unless your car requires it, doesn’t really offer enough benefit to justify the added price.

But what about petrol vs diesel? Based purely on price at the pump, diesel is cheaper. But that isn’t the full picture. Diesel vehicles have to pay road user charges, which can offset the cheaper fuel price. Diesel vehicles also tend to cost more, though tend to maintain their value for longer, too.

Working out which will cost less can be difficult, and depends on the model of car and how far you drive each year. But, once all the costs are factored in, the differences tend to be pretty small.

NZ Transport has a calculator you can use to help compare the costs.

Is charging an electric car cheaper than petrol?

For those who are really fed up with petrol prices, or are looking for a greener alternative, an electric car is always an option. Charging an electric vehicle is significantly cheaper than paying for fuel. However, from April 2024, all owners of pure electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) will be required to buy and display an RUC licence. The RUC rate for EVs will be $76 per 1000km unit – the same as for diesel vehicles.

The following table details the cost per 100km for three popular EVs, based on the RUC and national average electricity price of 33.74c per kWh. Do note that electricity prices vary, so check your individual plan to find your cost per kWh:

Car model kWh per 100km Cost per 100km
Tesla Model Y 14.6kWh $4.93 ($12.53 with RUC)
BYD Atto 3 16kWh $5.40 ($13 with RUC)
MG 4 14.6kWh $4.93 ($12.53 with RUC)

How does this compare to petrol costs?

By way of comparison, fuel economy is measured in litres used per 100km, and the following table details the cost per 100km for three popular brands of conventional petrol cars (based on a price of $2.82 per l).

Car model l per 100km Cost per 100km
Mitsubishi Outlander 8.3l $23.41
MG ZS 7.1L $20
Suzuki Swift 5.3l $14.95

The difference in fuel costs per 100km between EVs and petrol vehicles has narrowed. If you compare the average costs of running the EVs and ICE vehicles above, it works out to a difference of $6.76 per 100km.

Based on a travel distance of 14,000 km per year, the above averages work out to total yearly costs of $1776.60 for EVs and $2723 for petrol vehicles. This is a difference of $946.40. So while it’s still cheaper to drive an EV over a petrol vehicle, the introduction of RUCs for EVs has levelled the playing field considerably. Plus you must also factor in EVs’ higher prices: the Model Y’s starting price ($68,000) is around 60% higher than the price of a basic Mitsubishi Outlander ($42,000).

However, one way of boosting on-road savings is to sign up for an EV power plan, meaning you’ll be paying less to charge your EV.

EV power plans

These types of plans typically employ a day/night tariff structure, allowing for EV charging at night (usually between 9pm and 7am) when electricity rates are cheaper. This means you can simply plug in your EV at night and wake up to a full charge. Running your household appliances during the cheaper night tariff hours can also help cut costs. Keep in mind that higher rates will apply during the day.

If you do employ this strategy, the estimated savings could be even greater than listed above.

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About the author of this page

This report was written by Canstar Content Producer, Caitlin Bingham. Caitlin is an experienced writer whose passion for creativity led her to study communication and journalism. She began her career freelancing as a content writer, before joining the Canstar team.

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