What happens if I put petrol in my diesel car? Having recently exchanged her car for an Audi A3 2.0 TDI, my daughter made the mistake of filling it with petrol. The car was taken to an non-Audi garage, as instructed by her insurer. The repairer has changed the filter but not yet flushed out the fuel tank. Surely this must be a requirement? You can usually get away with up to 10 litres of petrol in a 60-litre tank. Fill up with high-lubricity diesel (such as Shell V-Power Nitro Plus) and keep doing exactly that. Don't allow the tank to be more than half-empty for about 2,000 miles. At first, the petrol will float on top of the diesel and won't get sucked into the low-level fuel pick-up. Then, very gradually, it will disperse into the diesel fuel. But if she put in more than 10 litres she needs to have the tank drained. If any of the petrol has reached the high-pressure injection pump, she will be in trouble. You can't fill a current Ford, BMW or Citroën diesel with petrol because the filler orifice will not allow it. Email your queries to letters@honestjohn.co.uk (please include your full name, postal address and phone number). More Honest John articles.