With retail gas prices hovering near $4, harried business travelers who don’t take a few minutes to fill up their rental car gas tanks before dropping off the vehicle at the airport are facing some daunting costs for refueling, according to a pair of surveys.

The newspaper USA Today checked with major rental companies to find out how much they charge per gallon for topping off the tank at 10 major airports when it comes back less than full – and it found prices as high as $9.30 a gallon. That’s how much Avis and Budget were charging at Boston, Dallas/Ft. Worth and Denver. Hertz wasn’t far behind, with a standard refill rate of $9.29 at all 10 airports.

Average gas prices in the metropolitan areas surrounding those 10 airports ranged from $3.84 to $4.32, the newspaper noted.

Enterprise Holdings conducted its own study of eight rental firms at 10 airports and is using the results to promote the relative affordability of its own subsidiaries – Enterprise, National and Alamo. The airport refill rates for those three companies were well below the competition, ranging from $5.63 to $6.27 a gallon.

The USA Today study also asked about the cost per gallon of buying the rental companies’ prepaid gas option, in which the renter pays in advance for a full tank of gas at the price set by the rental company. Those rates were much closer to the local retail price – but renters who buy the prepay option and bring the car back with a tank that’s not empty are essentially giving the rental firm free gas.