If having to pay $4 for a gallon for gasoline makes you sick then you may need to be revived to hear what some rental car companies are charging drivers who don’t pay ahead of time. If you don’t return your rental with a full tank, you may get gouged $9 or more for a gallon of gasoline.

As of April 25, Hertz (NYSE: HTZ) was charging $9.29 a gallon – the most on record – at the major airports to top off a tank when a car is returned without a full tank. With the average vehicle able to hold about 15 gallons, that could add an additional $83.55 to the rental car tab.

Avis Budget Group, Inc. (NASDAQ: CAR) charged less at $7.99 a gallon, and the lowest, $5.50 at privately-held Enterprise Holdings, was slightly more reasonable, considering the average price for a gallon of self serve gasoline is about $3.90. Customers who prepaid were charged between $3.57 and $4.28 per gallon, on average.

But rental car companies are getting away with charging the higher fees, especially at airports, because car renters often are business travelers unfamiliar with the whereabouts of local gas stations and little time to make their flights.

President Obama has promised to appoint a special task force to investigate possible price gauging at the filling station, but don’t expect the rental car companies to lower their prices as long as gasoline prices remain high. With daily rental prices as low as $40.57 for a mid-size car or SUV at Dollar Thrifty Automotive Group, Inc. (NYSE: DTG), and fewer customers opting to carry the rental car companies’ insurance, refueling fees contribute significantly to rental car companies profits. Last year, Hertz brought in $43.7 million in refueling fees, according to its most recent quarterly filing.

Most rental car customers probably aren’t charged the higher of the refueling fees, but rental car companies know that the threat is probably enough to lead many of their customers to accept the prepaid refueling offer. Americans already have cut back on driving because of the higher gasoline prices. Could corporate travel budgets be next?

As of this writing, Cynthia Wilson did not own a position in any of the stocks named here.