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    I have accidentally filled my fuel tank halfway with low grade petrol instead of high grade...

    I have accidentally filled my fuel tank halfway with low grade petrol instead of high grade. Can you please let me know what the consequences are?

    Answered by CarsGuide
    9 May 2022
    David Morley

    It rather depends on the car in question, Lee. If the car is designed to run on high-octane ULP (95 RON or higher) then you risk damage by feeding it any other grade of fuel. If you wanted to be completely sure that no harm would come to your 98-RON burning engine, then draining the tank and refilling it with the appropriate fuel is the way to go.

    In the real world, however, another vastly less messy and expensive option would be to fill the other half of the tank with the highest octane fuel you can find (preferably 98-RON) and then driving conservatively, avoiding high engine speeds or full-bore acceleration until you fill up completely with the right fuel next time. A modern car can often `sense’ what octane fuel is in it, and will adapt the engine to run on it without doing any damage. Some engines are designed to work most efficiently and make more power on a higher-octane fuel, but can cope with less octane if need be.

    If your car is designed to run on 95 RON, but you’ve half filled it with 91 RON, filling the rest of the tank with 98 RON should just about take the overall octane rating back to where it needs to be. The required fuel octane should be listed on a sticker inside the filler flap or listed in the owner’s manual.

    Disclaimer: You acknowledge and agree that all answers are provided as a general guide only and should not be relied upon as bespoke advice. Carsguide is not liable for the accuracy of any information provided in the answers.
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