Cornwall Council is raking in an extra #300,000 a year from motorists because its pay and display machines won't give change.
The authority made £11.6m in parking fees last year of which an estimated £307,293 was due to an "over-vend".
It made a further £340,000 from overpaying motorists the year before, according to figures obtained under the Freedom of Information Act.
The council claims that stocking its 250 pay and display machines with extra coins would be too costly.
A spokesman said: "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."