Lincolnshire Police forked out more than £4,000 after clumsy cops filled up with the wrong fuel.
Officers repeatedly re-fuelled their diesel vehicles with unleaded petrol, driving up costly repairs.
In four years between 2011 and October 2014, the mistake led to bills of £4,219.18.
In January 2012 one incident alone cost £2,432.33 – the most expensive incident in the four year period.
Fleet manager for Lincolnshire Police Nigel Coupe said: “Over the last five years Lincolnshire Police have accidently wrongly fuelled seven vehicles.
“The cost to equipping each individual fleet vehicle with a device to prevent such errors and the low number of such occurrences is not considered a good return on investment at the moment.
“Practical steps to reduce the risk of wrongly fuelling vehicles include improved signage on vehicles, a virtual 100 per cent diesel fleet and more emphasis to refuel at our own bunkered sites where using the wrong type of fuel is impossible.”
However, Dia Chakravarty, political director at the TaxPayers’ Alliance, said paying for the repairs was yet another strain on the taxpayer.
She said: “Mistakes happen, but the authorities must remember that when taxpayers are struggling to pay their bills, every penny counts.
“These costly mistakes mean less money available for essential frontline services. Lincolnshire Police must do better to minimise these incidents.”