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So a family member accidently filled a full tank of petrol in his diesel car.

He was able to get all of the petrol (bit of diesel) out of his tank by flushing it. Now he is left with about 40L of petrol or so with a bit of diesel mixed in it.

Would it be safe to use this mixed petrol in my petrol car?

Unable to determine the mix consistency but if I was to guess it would have to be maybe 10% diesel/90% petrol?

Are there any test strips or what would be a safe amount? As I could for example further "dilute" the mix by adding small quantities with my normal fills (let's say about 5L with my normal weekly 30L fill.

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Diesel fuel does not directly harm the materials and equipment in a petrol engine, but adding diesel to petrol will dramatically lower the octane rating.

Diesel fuel has a cetane rating which is functionally the opposite of octane rating. Pure diesel fuel has an octane rating somewhere between 25 and 40. Petrol sold at the pump runs from the upper 80s to lower 90s. (Generally 91 or higher in the UK, 87 or higher in the USA.) Each 2% of diesel added to petrol lowers the octane rating of the mix by about 1 point. So if you really have 10% diesel in the petrol, the octane rating has been lowered by about 5 points. But remember that a fuel gauge that reads empty is usually lying, and there's more fuel remaining in the tank than indicated. What if the mix is really 20% diesel? Octane would be reduced by 10 points.

If you fill your tank with this stuff, it will run like sh.... uh, it will run poorly. The engine will knock. Although all modern petrol cars have a knock sensor to deal with variations in octane rating by altering the timing, a drop of 5 to 10 points is really too much for the ECU to deal with, and it's possible for excessive knock to damage the engine. This is especially true of you have a high performance, high-compression engine that requires high octane fuel to run at peak performance.

In my opinion, your proposal to mix 5 liters of diesel-contaminated fuel with 30 liters of pure petrol per week is a reasonable approach that will greatly reduce the likelihood of adverse effects, especially if you buy the highest octane petrol available during this period. It may be costly, but certainly less costly than paying a company to dispose of the contaminated fuel.

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    Another option is to put a couple of litres into the diesel tank when it is refilled (from near empty). It was once common practice to put in a bit of petrol in the winter, to act as an antifreeze. Commented Jul 10, 2023 at 12:47
  • @WeatherVane Excellent idea! Around here in New England as late as the 1990s, filling stations routinely mixed 1:15 gasoline in their diesel fuel in the winter. Hopefully the petrol in the OP's region is as ultra-low in sulfur as the diesel fuel is.
    – MTA
    Commented Jul 10, 2023 at 12:54
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    Or, as your recent answer to a similar question: use it up in garden equipment (though it might take a while to get through 40 litres). Commented Jul 10, 2023 at 13:05
  • The filing stations diluted the diesel with gasoline? Was that in their storage tank or the customer's tank? If the former, with diesel typically more expensive, did they still charge the full diesel price? Commented Jul 10, 2023 at 20:58
  • @WeatherVane That's right, the filling stations mixed diesel and gasoline in their storage tanks in winter. While filling your diesel fuel tank on the vehicle, you'd pick up a whiff of gasoline fumes coming out of the filler neck. This was in Vermont and strange to me, as I was living in NYC at the time, where that wasn't done. I asked the attendant about it and he said, "Ayup, fifteen to one so it don't gel." I don't remember if the price was different, but in 1990 diesel was $0.73/gal and regular unleaded was $1.16/gal.
    – MTA
    Commented Jul 10, 2023 at 21:33

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