Fuel grades: Why can’t I choose the cheaper one?

September 2, 2022

Customer refuelling their ute at a Z service station

Ever stood at the petrol pump and wondered if your car really needs the higher octane fuel?  

When times are tough, it’s natural to look for savings, but before you reach for the lower octane - and lower-priced fuel - make sure you understand what you’re putting into your vehicle. 

Simply put, it is vital you use the fuel recommended by your car manufacturer. Using the wrong fuel can cause damage to your vehicle, lead to dangerous situations on the road, and it’s more costly in the long-run. 

So, if your car requires 91 octane then you can use that, or if you're happy to pay a little more for enhanced performance, you can use 95 petrol. 

The advantage of higher octane fuel is a greater stability under pressure – it is less likely to combust in the chamber before it’s supposed to do so. 

Remember, the golden rule is to use the recommended octane as a minimum. While it’s always tempting to save money, if your car requires 95 octane, you must not use 91. Check your car’s manual, or the inside petrol flap, for guidance on which fuel is best for your vehicle or fleet. 

But what do the fuel grades mean? 

You will have noticed different fuels have different names and numbers attached. Simply put, these correspond to their research octane ratings. The ratings are an indication of how well the fuel resists burning too early inside the car’s engine. In New Zealand the minimum research octane number must be displayed on the dispenser pump. 

At Z, there are three types of fuel to choose from: Z91, ZX Premium Unleaded and Z Diesel 

When it's 91 octane you need, choose Z91 Unleaded. It includes a multifunctional additive package sourced for Z petrol, keeping your engine clean and protecting your fuel systems against corrosion. The good news is a clean engine can help improve vehicle drivability and reduce emissions. 

For cars that require minimum 95 research octane fuel, ZX Premium Unleaded also contains a high quality specially formulated multifunctional performance additive. This Z95 unleaded fuel is designed to have superior engine cleaning capabilities so the engine can perform at their best. 

Z Diesel is a special purpose fuel for use in high-speed diesel engines (diesel engines operating at speeds above 800 rpm). Z Diesel is well suited to be used in private vehicles as well as commercial fleets and for marine and industrial applications. 

While it’s not a fuel, it’s important to mention Z’s emission-reducing AdBlue - a diesel exhaust fluid used in vehicles with Selective Catalytic Reduction (SCR) systems. A growing number of manufacturers are employing this technology to help their modern trucks, buses and other diesel vehicles to reduce emissions and meet stringent Euro 4, Euro 5 and Euro 6 emission standards. AdBlue is kept in a separate tank in each vehicle and added to the engine exhaust gas after combustion has occurred. Remember, this is not a fuel additive and must not be put directly into a diesel tank as this can seriously damage the vehicle. 

What happens if I put the wrong grade fuel in my tank? 

If 91 is used in cars requiring higher octane fuel, you run the risk of pre-igniting or "knocking". This occurs when the petrol combusts early and pushes down against a piston while it is still moving upwards during the compression stroke. This is noticeable as a knocking sound in your engine and can cause major damage if left unchecked.