Some rental car companies are charging as much as $13 a gallon when consumers return a car on empty, though not anymore in Maryland.

Most car-rental companies offer consumers the opportunity to buy a full tank in advance, then return the car with the same amount of fuel as when the vehicles was rented. Consumers can also choose to have the agency refill with gas priced at a premium over the market rate.

When the car come comes back to the rental company on empty, the gasoline bill can sometimes be be a shock, depending on the rental agency.

AAA found one rental company near Philadelphia International Airport charging $13.50 a gallon for refueling.

Budget Rent-A-Car has charged as much as $9 a gallon for a refill and Hertz has charged as much as $7.99. One consumer reported Rent-A-Wreck charged $7 a gallon plus $10 if the car came back on empty.

Maryland Attorney General Douglas Gansler negotiated an agreement with car rental companies in his state to reduce refueling charges following a six-month investigation of gasoline prices charged consumers by the rental agencies.

"Marylanders are already hurting at the gas pump, and paying $8.00 per gallon is just salt in the wound," Gansler said. "We have made it easier to visit and do business in Maryland, and these agreements can be a national model for states that want fair pricing."

The Maryland investigation included rental charges at Baltimore/Washington International Thurgood Marshall Airport and disclosed that some rental car companies had increased refueling charges far in excess of gas prices found in the Baltimore/Washington/Philadelphia area.

Prior to the agreement, consumers would have paid an $8.00 per gallon refueling fee. Now the cost is a $5.85 per gallon refueling fee or 140 percent of the prevailing price of full service fuel.

AAA said car-rental companies across the country should cut refueling charges.

"It is ludicrous for a car-rental company to charge more than twice the price of a gallon of gas, especially when motorists are already paying a lot for gas," AAA spokeswoman Catherine Rossi said.