Premium petrol costs 15–25 cents a litre more than regular 91. Whether that’s money well spent comes down to one thing: what your engine was designed for. The octane number is a requirement, not a quality scale.

What the numbers actually mean

91, 95 and 98 are RON (research octane numbers). Octane measures one property only: how much compression the fuel tolerates before it ignites on its own. Higher-compression engines need higher octane to avoid uncontrolled ignition (“knock”), which can damage the engine over time.

Octane is not purity, power, or cleanliness. 98 is not “better fuel” than 91 in some general sense. It’s fuel for engines that need 98.

The only rule that matters: check the flap

Open your fuel flap or your owner’s manual. It states a minimum RON.

What your fuel flap's minimum RON means for which fuel to buy Check the fuel flap 91 or higher Use 91 95 minimum Use 95 98 recommended 95 minimum 95 OK, test 98
What the flap says decides the fuel: 91-or-higher cars buy 91, 95-minimum cars buy 95, and 98-recommended cars are safe on 95 with 98 optional.
  • “91 or higher”: use 91. Your engine was designed for it. Putting 95 or 98 in delivers little to no measurable benefit for most cars in this group. Modern engines can adjust slightly, but the gain rarely covers the price gap.
  • “95 minimum”: 95 is the floor, not a suggestion. Using 91 risks knock; on many modern cars the computer protects the engine by pulling performance back, so you pay less at the pump and get a worse-running car.
  • “98 recommended, 95 minimum”: 95 is safe; 98 may add a little performance or economy. Whether it’s worth it is measurable: run a few tanks of each and compare.

“But premium has better additives”

Premium blends often carry more cleaning additives, and brands market that hard. All petrol sold in Australia must already meet national fuel-quality standards, and the additive difference is far too small to justify a 15–25 c/L premium on a car that needs 91. If the flap says 91, the cheapest 91 nearby is the right buy.

Wondering about E10? That’s a separate question. E10 is 94 RON with ethanol, and we’ve covered the real cost comparison separately.

The dollars

A 91-car filled with 98 pays roughly $10–12 extra per 50-litre tank for close to nothing. Weekly, that’s over $500 a year: several times what any docket scheme saves, wasted in the other direction. Meanwhile a 95-car filled with 91 saves $3–5 a tank while quietly running worse.

Match the flap. Then spend your effort where the real money is: buying at the right point in the price cycle, at the cheapest station near you.