What Happens If You Put The Wrong Octane Of Petrol In Your Car

I drive a lot of different vehicles when I need to get around, but I'm always a little worried when it's time to fill them up. Will something happen if I use 91 instead of 95, or vice versa? Well, this thread at StackExchange answers the question, and long story short, there's little to worry about. Photo by Mike Mozart.

Of course, the thread is concerned with a situation where, for example, you accidentally put high-octane in your tank. Ultimately, it's nothing serious to worry about (and not nearly as serious as putting unleaded into diesel vehicles or vice versa, which may necessitate you empty the tank):

Higher octane fuel does not burn as easily as a lower octane fuel. Higher octane fuels are specified where higher compression ratios are present in an engine, or where forced induction (such as turbo charging) is used. By using higher octane fuel where lower is specified, you will create no problems with your engine. It does not (by popular belief) add any power to your engine, but will not harm it at all. Worst case scenario is you've just wasted some money by buying the more expensive fuel.
If you put the lower octane fuel in an engine which specifies high octane fuel, will not cause you any major issues on an electronic fuel injected engine because it has a device known as a "knock sensor" which will pull timing.
...When you mix different octanes of fuel, you are either increasing or decreasing the octane of the fuel at hand. It won't cause any real problems for the engine or fuel system at hand (this assumes you are using fuels of the same mixture of ethanol -- mixing E85 fuel into standard fuel to increase octane and introducing it into a fuel system which cannot handle it - read this -- may cause issues with seals and corrosion of parts which are not built to take the higher concentration of ethanol. E10 fuel poses no issues for modern or older vehicles).

The full answer goes into much deeper detail on the knock sensor and how it works, and discusses what would happen if you did introduce diesel into a tank with unleaded in it already.

Either way, if you've ever wondered like I have (and flipped through the owner's manual in a car you were renting to make sure you got it right -- which you should do anyway, to be honest, if you're driving an unfamiliar vehicle), you can rest knowing you won't hurt the car -- but it's always a good idea to get it right for efficiency's sake.

What happens if you put the wrong kind of gas in your car? [StackExchange]


    Is that only in relation to the 91/95/98 stuff, versus E10? Though I thought 91 and E10 were basically the same thing, and apparently they're not.

      E10 in Aus used to be 91 + 10% ethanol, but recently some providers have made 94 + 10% ethanol their E10 mix. 91 in some stations will be just plain 91 octane, or it might be E10.

        Since when can they sell you ethanol based fuel without labelling it?

          It is labeled. But servo 1 will sell 91(e10), 95, 98, servo 2 will sell 91, 94(e10), 98, servo 3 will sell 91(e10), 98 etc.

          E10 isn't always 91, and 91 isn't always E10.

            Ah yea, I just drive on. Mind you it has been a while since I've had a vehicle that I didn't put 98 in, but I remember when the spare car was an 88 Barina, poor little thing would ping it's goddamn head off with 91e10, you just can't do that to a carby.

            I feel sorry for all the people with cheap or old carb motorbikes too.

        I thought the 9X numbers were percentages too. What do they actually mean then? Unless we're getting 104% petrol :P

          Research octane number. https://en.m.wikipedia.org/wiki/Octane_rating

    There is no way I'm putting E10 in either of our family's cars.

    This has nothing to do with eco-. It has to do with the fact that you save a couple of cents per litre, but your corresponding drop in fuel economy (and performance) ends up costing you more than you save. If 91 is not available (as in parts of NSW), I'll happily put 95 or (at a pinch) 98 before I stick E10 in the tank.

    THINKS: I miss the days when I used to put 1 gal of Methyl-Benzine to 4.5 gals of Super in each tank of my 1275 Cooper S, or better yet, 100-130 AvGas if you could get it. Ahhh, mis-spent youth, where are you? :-))

    Last edited 26/08/16 10:26 pm

    True point on E10 (lower energy content per litre more than wipes out cost savings). (source: I was once an oil refinery engineer). E10 would need to be a lot cheaper to be worth buying.

      Unfortunately they don't really have to make it cheaper when they replace an existing option in the lineup.

    Gooky, E10 is petrol containing up to 10% ethanol by volume. Hence it's chemically different to regular petrol. Refiners like to use E10 as it gives a nice boost to octane number without the high price of alternatives. Downside is it lowers your fuel economy due to ethanol's lower energy content.

    E10 poses no issues on new or older cars?
    Au3 falcon. Severely reduced economy. But no other effects.
    XD falcon. Complete fuel system clean, carburetor clogged.
    Mazda 323 GT. Reduced power. Probably due to knocking because of lower octane base fuel.

    Once again, another article falsely claiming that using a higher octane than recommended does nothing. This is blatantly false in most circumstances, especially so these days. The number recommended is the minimum octane rating that the manufacturer warrants will not damage your car over long-term usage. In modern cars (anything since, oh I dont know, the mid 90's) using a higher number than this can help increase fuel economy and power delivery, as the ECU will modify paramenters to reach best performance past the "default" RON settings.

    To pick a first-hand example:

    2011 Mazda 2
    91 (E10) - Worse $/km of any petrol
    91 - Recommended RON / "Control"
    95 - Gets best $/km out of any fuel, noticably more perky than 91
    98 - Marginally better economy than 95, costs more obviously, noticably more perky than 91

    I want to be sure if e10 is good for my Pajero np 3.8 lt thanks

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