"Should I be putting the expensive stuff in?" is one of the most common questions drivers ask when they are standing at the bowser choosing between 91, 95 and 98. It feels like the higher number must be better — but that is not how octane actually works.
Those numbers are octane ratings, specifically the Research Octane Number (RON). In Australia, regular unleaded must be a minimum of 91 RON and premium unleaded a minimum of 95 RON, with 98 sitting above that — the parameters are defined in the government's petrol fuel quality standard. This guide explains what the number means and whether paying more for a higher one is actually worth it.
What Is an Octane Rating?
The octane rating measures a fuel's resistance to premature ignition, known as engine knock or detonation. In a petrol engine, the air-fuel mixture is supposed to ignite only when the spark plug fires at the precise right moment. If the fuel ignites too early due to compression heat alone, it causes a knocking sound and can damage engine components over time.
Higher octane fuel resists this premature ignition better. That is its entire purpose — it is not about energy content, cleaning ability or power output per se.
RON 91: Standard Unleaded
RON 91 is the minimum octane petrol sold in Australia. It is suitable for naturally aspirated engines designed to run on standard fuel. The vast majority of older Australian cars, many Toyota Corollas, Mazda 3s and similar models run perfectly on 91.
RON 95: Premium Unleaded
RON 95 is specified as the minimum for many modern turbocharged engines, European cars and some performance variants. These engines operate at higher compression ratios or boost pressures, so they need fuel that resists knock under greater stress.
RON 98: Ultra Premium
RON 98 is recommended or required for high-performance and luxury vehicles. Cars like the BMW M3, Mercedes-AMG models, and Porsche 911 are tuned to take advantage of 98's knock resistance, allowing the engine control unit to advance ignition timing for peak power and efficiency.
Will Premium Fuel Help a Standard Car?
If your car specifies RON 91, filling with 98 will not add horsepower or improve fuel economy in any meaningful way. The engine management system is calibrated for 91 and cannot advance timing beyond its design parameters. You are simply paying more for a property your engine cannot exploit.
However, if your car specifies "91 minimum, 95 recommended" — a common specification — then using 95 may yield a small improvement in efficiency and responsiveness, as the ECU can optimise timing with the higher-octane fuel.
When Premium Fuel Is Essential
If your owner's manual states "minimum 95 RON" or "minimum 98 RON," you must use that grade or higher. Modern engines with knock sensors will retard timing to protect themselves when lower-octane fuel is used, but this reduces power and efficiency. Prolonged use of inadequate octane can cause cumulative engine damage.
The Additive Question
Premium fuels sometimes contain better detergent packages that help keep fuel injectors and intake valves clean. Some drivers report smoother running after switching to premium for this reason alone. However, all Australian fuels must meet the minimum standards set by the Fuel Quality Standards Act, so the base-grade fuel is not leaving your engine dirty.
Practical Advice
Check your fuel flap or owner's manual. Use the minimum specified octane or higher. If your car says 91, save money and use 91. If it says 95, do not go below 95. And if it says 98, treat that as a non-negotiable requirement for the health of your engine.