No Way Around It, Pay Now or Pay Later to Refill Car : Rentals: Advance-purchase gas programs might look good on paper, but they could cost more.

It’s not logical, but it happens: Bedraggled consumers spend their springs scrambling after cheap air fares and chasing down affordable hotel rooms and rental car rates. Right about now they leave on their vacations. And in a week or so, on the final day of their trip, they discover A) that they’re running behind in returning the car to the airport; B) that their gas tank is far from full, with no gas station within convenient distance; and C) that they are going to have to pay their rental car company nearly $3 a gallon to fill the tank with gasoline that elsewhere costs less than half that much.

For years, this has been a built-in part of the rental car agency business, in which Hertz, Avis and their brethren penalize drivers for returning cars with tanks less than full. (For some figures on who charges how much where, see below.)

But in the last few years, reservations personnel at Hertz, Avis, Alamo, Budget, Dollar, National and other rental companies have been offering another option to drivers who don’t want to be bothered with refilling their tanks.

It could save you money, but it’s a somewhat perverse program, and travelers should consider it carefully before using it.

The program, usually labeled “fuel service option” or “gas service option,” is based on the idea of advance purchase. Renters are given the option of paying up front for gas that may be needed to refill their tanks later. In exchange for handing over their money early, renters get a much better rate per gallon. But the renters also waive the right to any refunds for the gas that is left in their cars’ tanks.

For instance, an Avis renter in Denver, returning a car last week without refilling the tank, had this choice: Agree in advance to pay for a tank refill at $1.07 per gallon and, in order to get maximum value, return the tank as close to empty as possible; or pay nothing up front and face charges of $2.69 per gallon at the Avis office if the car is returned less than full.

But if that renter chose the advance-purchase program and returned, as a prudent driver should, with some gas remaining in the tank, the cost-effectiveness calculations get complicated. Use the advance-purchase program and return a car with a 10-gallon tank one-quarter full, and you have effectively paid $1.43 per gallon; return with half a tank, and you’ve paid $2.14 per gallon; return with the tank three-fourths full and you’ve effectively paid $4.28 per gallon. It seems unlikely that rental car agencies will be losing money in any of these transactions.

Further, at a time when so much attention is focused on the safety of rental car drivers, it seems strange to encounter an industry-wide program whose economics encourage driving around in a strange city with as little gas in your tank as possible.

Rates vary from company to company and city to city, as the following week-old figures from rental agency airport locations illustrate:

Avis in San Francisco charges $1.14 per gallon in advance, $2.90 otherwise; in Denver, $1.07 in advance, $2.69 otherwise; in Albuquerque, $1.03 in advance, $2.59 otherwise. Hertz in San Francisco charges $1.16 in advance, $2.84 otherwise; in Denver, $1.11 in advance, $2.69 otherwise; in Albuquerque, $1.07 in advance, $2.59 otherwise. Rates are generally comparable at other companies.

The economics will vary depending on prevailing rates at service stations. But for my money, making time to stop and top off the tank at a gas station, and then returning the car, is still easily the wisest alternative.

Drivers should remember, however, that gas station operators are working their own angles on this issue. In Denver, an Avis clerk reported that prices around town were running about $1.12 per gallon of self-service unleaded gas but that a prominent gas station near the rental car office was asking about $1.45. (Things are tough all over for travelers renting cars in Denver; rental cars there carry extra fees of $2.98 per day to help cover costs of the city’s new airport.)

Another caution: Renters should be certain that reservations clerks understand whether they want or don’t want the advance-purchase program. One colleague of mine took the trouble to refill her gas tank before returning a rental car, only to find that the company had enrolled her in its advance-purchase program and was charging her the full amount despite the full tank she’d brought them. (Fortunately, the reservations clerk promised credit on a future rental.)