The manager of an Alexandria gas station has been fined $250 for overcharging gasoline customers as much as six cents per gallon.

The fine was levied in the city's General District Court on Tuesday after Alton C. Morgan, manager of the Sunoco station at 4510 Duke St., pleaded guilty to a charge of misrepresenting the price of gas.

Howard J. Holton, director of the city's Finance Department, which investigated the incident, warned consumers "to always carry a little calculator with you. You just can't tell by the numbers on the pump" what the real charge should be.

The misdemeanor charge against Morgan centered on a 30-cent difference in price between the reading on a pump meter and a calculation made by City Inspector Louis W. Vezina, who made a routine check March 13. Vezina said he didn't know whether the difference was the result of intent to defraud.

Morgan could not be reached for comment yesterday, but Bob Watson, divisional sales manager for the Sun Oil Co., which owns the station, said Morgan had told him the problem was due to a mechanical failure and he had pleaded guilty "only because he was responsible for the pumps."

Vezina said that after discovering the overcharge he checked with other-local jurisdictions and with state officials in Richmond. "they said they'd never encountered anything like that before," he said.