Car Advice

Myth-Busting: You should use higher octane fuel…

Higher octane fuel costs more than regular unleaded, but is it worth using 98RON if your car is recommended for 91RON, many say that you should. So, should you?

TV ADS and billboards proudly proclaim that pumping the highest octane fuel into your car will be like a “fitness programme for your engine”…that’s what I saw one petrol company claim about its 98RON fuel. And that’s true, if the maker of your car stipulates it’s been tuned for 98RON fuel. If not, then while you’re doing no harm, you’re not really helping your car either.

Sure, premium unleaded (98RON) is undoubtedly a better fuel in the right engine and it does offer more power and burns cleaner in engines tuned for this type of fuel but if you’re engine’s not specifically intended to run on it, then many of those benefits won’t be realised. So, that means pumping premium unleaded into your 1984 Toyota Corolla is a complete waste of money.

See, most modern cars run a specific compression ratio which is a measure of how much room there is available for fuel when the piston is at the bottom and top of the cylinder. Indeed, your average petrol and diesel powered car, thanks to a fairly common compression ratio of, say, eight to one (8:1), is designed to tolerate lower octane fuels.

That means, according to William Green, a chemist at Massachusetts Institute of Technology “the regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money”.

Where premium unleaded does become necessary, is when you have a higher compression ratio, say, in a turbocharged vehicle. Most car makers will specify on the fuel flap what type of fuel the vehicle is happy to run on, and if it says 95RON, then there’s absolutely no reason to run 98RON in it. If, however, the maker stipulates premium unleaded then you absolutely should shell out the extra coin for the heady brew.

People often talk about an engine knocking if the wrong fuel is used, a knock is an uncontrolled explosion and the occasional one won’t hurt your engine. More than that, most cars built after 1996 carry a knock sensor which is a clever little device that can detect knocking and tweak the timing of the spark plug to cancel it out.

What about marketing claims of premium fuel providing more power? Well, that’s only true, again, if your car’s specified to run on premium unleaded. It means the maker has designed the cylinders to produce more pressure and thus extract more power from the fuel. Sadly, if your car is tuned to run on regular unleaded then pumping premium into it won’t extract more power. Sorry.

What about if you open up the fuel filler flap and, like the one in the photo it just says Unleaded Fuel Only…well, you’ll need to open up your owner’s manual to find out the recommended fuel.

This article is a cut-down version of a much longer one detailing knock, RON and other such nerdiness, you can read that one HERE.


Isaac Bober

Isaac Bober

Isaac Bober was born in the shadow of Mount Panorama in Bathurst and, so, it was inevitable he’d fall into work as a motoring writer. He began his motoring career in 2000 reviewing commercial vehicles, before becoming editor of Caravan & Motorhome magazine. He then moved to MOTOR Magazine before going freelance and contributing to Overlander 4WD, 4×4 Australia, TopGear Australia, Men’s Health, Men’s Fitness, The Australian, CARSguide, and many more.