Parking machines that do not give change are making more than £300,000 a year for Cornwall Council. a Freedom of Information request has highlighted.
While £11.6m was paid in parking fees, an estimated £307,293 was due to an "over-vend", where people do not have the correct change.
However Cornwall Council says that stocking the machines with change would create 'logistical problems, and lead to higher costs.
The council said: "In Cornwall, there are over 250 pay and display machines in locations from Bude to Porthcurnow and if these machines gave change there would have to be a mechanism to ensure they were continually stocked with sufficient change and to address the situation if they were not.
"The logistical problems of doing this would be great and would result in a high cost.
"The 'base level' of change would also be left in the machines at all times making them much more prone to vandalism and theft."
"The amount of overspend only represents about two per cent of our total income from car parking charges," it said.
"Like any surplus parking income, it is used to support transportation requirements such as road maintenance and concessionary bus fares."