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.

  • Alan

    I did extensive comparisons (2-3 fills in a row) in my 2006 (car with knock sensor, CVVT) with my car where I did the same commute day after day. Yes 95 and 98 gave better economy – but not enough to warrant the e$tra $$$. It only would have been viable if the price differential was ½ of what we’re charged for 95/98.

    The other thing which was obvious was that with E10, it used more fuel – much more than was compensated for with the cheaper E10 price.

  • Ben Tate

    I agree with Alan. E10 needs to be much cheaper to compensate for the loss of economy when using it.

    Some say the ethanol in E10 is a great fuel system cleaner. Others suggest it might gum things up. It’s hard to know who to believe.

    Theres no point bunging 95 RON in a lawnmower but there’s no choice when the mower can’t use E10 and there’s no 95 RON. The upside of 95 RON is that most have a good dose of fuel injector cleaner.

    Didn’t the Govt tighten up the rules on the definition of petrol? My old VP Commode ran like a bucket of bolts after a fill from an independant servo. A top up with 95 RON diluted the Toluene brew to the extent that the Check Engine light went off.

    My Kwaka 1200 cannot burn E10 due to rubbers used in the fuel system I guess. Its happy on 91 RON but I use 95 RON assuming that the fuel system cleaners help to keep it running smoothly.

    I wonder if using 95 RON in an engine designed for 91 RON causes a loss of power? Fuels with a high RON No. burn more slowly.

  • Garry Koppen

    I understand that turbocharged engines benefit from 95 or 98 use
    now the Hyundai i30 SR range with the turbo 1.6 petrol engine is stated as being able to run on 91
    Will this and similar cars gain from using 95 or 98 ?

    • Hi Garry, with a turbo’d car, yes, running a higher octane fuel will likely have some benefit but despite the anecdotal evidence of others, I’d suggest the benefits will be four-fifth of … all. Hyundai has tuned the car to run on 91, and I run 91 in the one I’m driving, and have alternated it for a month at a time… and I didn’t get any better mileage.

      • Garry Koppen

        Thanks for that , one further thought ; what about sulphur content which is a lot higher in 91 fuel ?
        Talking about reliability and longer engine life and lower maintence costs not fuel economy or power

  • Graham Patterson

    I continue to see articles such as this which I can agree that in a perfect world there is no consequence to using low octane, or ethanol blends of fuel. One huge problem is that increasingly owners do not operate their vehicles in perfect operating conditions. Given the extended oil change periods the manufacturers now suggest is good for their vehicles without emphasising the page on operating under harsh conditions, we now see vehicles in a very short period, developing carbon build up in the combustion chamber causing an increase in combustion ratios well past that which they were meant to operate comfortably on with low octane fuel. in the last 10 years or so, the results of this is becoming more evident in the industry, and costs to repair something that can be minimally addressed by using higher octane fuel, or slightly higher maintenance procedures such as Subaru suggest on every service, of performing an upper end clean, the bigger expense replacement of head gaskets due to hotspots due to lower octane fuel detonation are becoming far more frequent. This is a repair that was exclusively due to an old, or poor inhibitor situation that produced head gasket failures of the past. In EVERY situation in recent years of a breach of combustion chamber pressure from a failed head gasket, the evidence is deterioration resulting in the gasket burning from the combustion chamber out, not caused by the coolant failure causing the gasket to corrode inward, confirmed in every case by these customers commenting they only use e10 or low octane fuel.

  • Guest

    My “grocery getter” requires premium unleaded but the cars won’t visit the petrol station of 7-8 weeks coz low k’s and interchange between 2 cars.

    Not wanting fuel to go stale, should I use 98 or stick to 95?

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.