We can get three types of petrol in New Zealand: 91, 95 and 98 RON (Research Octane Number). They’re all types of unleaded petrol (ULP), but 95 and 98 are often called premium unleaded petrol (PULP) or super. If you’ve driven in other countries you might have seen 90 octane in Japan, 97 octane in Malaysia or America, or 99 octane in the UK. The fact we use 91, 95 and 98 are not international standards.
Octane rating is the fuel’s ability to resist uncontrolled ignition or auto-ignition which causes engine knock at high compression ratios. Basically, this is when the spark doesn’t ignite the fuel at the right time in the piston which causes more pressure in the piston but less actual power transferred to the road; it’s not good for the engine and can cause a metallic pinging sound.
Some manufacturers suggest that you should use premium unleaded rather than 91-octane unleaded, and petrol manufacturers claim that these fuels will help clean your engine, protect it and give you better fuel economy. Premium fuels cost more per litre than regular unleaded, though.
Should you use premium unleaded even if you don’t need to, and can you use regular 91-octane unleaded even if the manufacturer says you should use 95 or 98?
If you have an unmodified family car (i.e. not a performance car), and you never give it a thrashing, it’s unlikely that you will notice any difference if you put premium unleaded in it. If you do like to drive it hard, you might find it a bit more responsive as the revs climb, but it’s unlikely to be much more responsive.
If you have a high-performance car and you put low octane fuel in it, but you drive gently, it’s unlikely that you will notice any difference if you put regular 91 octane unleaded in it.
If you have a high-performance car and you want to use its performance, you will (from experience) notice a difference. There will be less power, and you will probably notice ‘pinging’ from the engine if you stick your foot down because the fuel wasn’t controllable enough to prevent premature detonation.
Should you use a tank of premium unleaded every so often, even if you usually take regular unleaded?
Yes, this is perfectly acceptable and you might get the advantage of some extra engine cleaning which could improve your fuel economy. However, there are so many factors that affect fuel economy (read about them here), that it’s unlikely you will find out unless you run repeated, standardised tests.