The Police force in Avon and Somerset has admitted spending £23,000 fixing vehicles filled with the wrong type of fuel in the past three years.
Since 2005, more than 100 cars have been put out of action by civilian staff and officers who put petrol in a diesel tank or vice versa.
A spokesman told the BBC that the constabulary was looking at ways to reduce the problem.
'It happens about 50 times a year,' he said. 'That sounds a lot, but we actually fill up about 57,000 times in total.'
Figures released under the Freedom of Information Act show that the force spent £8870 on wrong-fuel repairs during the financial year 2007/8.
The year before, the cost was £8353, and the 2005/06 bill was £6074.
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