Avon and Somerset Police spent £23,297 during a three-year period on repairing police vehicles after employees filled them with the wrong fuel.
Since 2005, 148 vehicles have had to be repaired because staff had put diesel in petrol tanks or vice versa.
The figures, released under the Freedom of Information Act, show the biggest cost to the force was in 2007-08 when it spent £8,870 on repairs.
An Avon and Somerset Police spokesman said: “We have tried various aftermarket devices, including a unit which warns the driver when they open the fuel flap, to try to reduce the incidence of this occurring.
“But these are not very successful and some forces have found them to create more problems than they solve - blow back of fuel and very much extended re-fill times for example.”
News of the cost to Avon and Somerset Police comes weeks after Essex Police revealed it had spent £40,000 over a five-year period repairing vehicles, which were also filled with the wrong fuel.
“Eventually, motor manufacturers have recognised and responded to the issue,” added the police spokesman. “Ford, for example, now fit a device to their Mondeo and Focus ranges as standard. I therefore anticipate this problem will begin to drop away.”