Unlimited Mileage--an Idea Whose Time Is Running Out : Rentals: Car agencies are quietly restricting free-mileage packages. But deals vary widely, so it pays to shop around.

Unlimited mileage . Surely, those are two of the most seductive words you can whisper to anyone contemplating a road trip in a rental car. But leading U.S. rental car companies, looking to cover increased overhead costs, have stopped their whispering and started experimenting with cutbacks in mileage allowances.

This means that renters, many of whom have already noticed modest hikes in basic daily rates over the last three years, may now find themselves paying an added 20-25 cents per mile once their travels take them more than 100 miles per day.

Some companies, such as Alamo and Hertz, have tried to aim these new fees at hometown renters (who tend to rack up more miles and liability) and continue to offer unlimited mileage to renters who arrive by air and rent standard cars. But other companies, including Avis, are moving toward blanket policies that impose mileage fees whenever a renter averages more than 100 miles daily.

Most industry insiders trace the trend to Alamo Rent a Car, which staged a now-completed free-mileage-cap experiment in April and May, or Budget Rent a Car Corp., which last year tried mileage caps in some markets. But now all the biggest companies are in on it, though they’re keeping mum about many details as they phase the fees in. Thus, if you’re planning to rent a car anywhere in the United States this summer, a call to the company and some close questioning is a good idea.

In some cases, companies are imposing the caps first on minivans, convertibles and Lincoln Town Cars, and not yet including economy and compact cars. Some companies will still give renters unlimited mileage, but only if the renter is willing to pay a substantially higher daily rate. A mid-size car from Hertz’s LAX location now runs $53.99 a day with a mileage cap, $103.99 a day without.

Why? Through the 1980s, while American auto makers were suffering through years of rotten retail sales, they sold large numbers of cars at low rates to this country’s rental companies. But over the last three years, as American cars have become more popular with the buying and leasing public, car makers have been selling fewer vehicles to rental companies--and at higher prices. As a result, the biggest rental companies (many of which are at least partly owned by Detroit car makers) have been leaving their new vehicles in service longer--which means that when they sell the used vehicles back to the manufacturers, as most car rental companies do, they’re now getting lower prices. The rental companies have raised basic daily rates slightly, but not enough to make up the difference.

Here’s a sampling of mileage policies at some of the nation’s largest rental car companies.

Hertz, which started moving toward limited-free-mileage rentals in April, now has them in place in 52 cities. Spokeswoman Lauren Garvey said the program is currently aimed at non-fly-in renters, and that no limits have been imposed at San Francisco International Airport. At Hertz’s LAX office, mileage caps are in place for non-fly-in renters, Garvey said. (When customers call to make bookings, reservationists typically ask renters if they will be arriving by plane.) The charge for travel beyond 100 miles per day is generally 25 cents per mile, 29 cents in New York.

Avis made its initial rollout of free-mileage limits in Detroit and at New York’s Kennedy Airport in May, and on July 13 it is planning to add caps for both fly-in and non-fly-in customers at 21 more locations, including San Francisco but not in Los Angeles or Orange counties. The charge beyond 100 miles per day: 25 cents per mile. Alamo tested caps on non-fly-in renters in about 75% of its markets (including Los Angeles) in April and May, spokeswoman Sandy Richards said, and then discontinued the test. Richards expects a decision on what comes next by mid-August. The Alamo test, which covered all types of vehicles, allowed 100 miles per day and charged 20 cents per mile after that.

Budget last year began testing free-mileage caps on minivans, Lincoln Town Cars and other specialty vehicles in “the central U.S. region.” In July, the company plans to expand the program to specialty cars in several new markets--but as of June 6, Budget hadn’t disclosed which cities. Drivers who average more than 100 miles per day will be charged 20-25 cents per mile (probably more in New York) for the extra distance, a spokeswoman said.

National Car Rental started experimenting late last year with free-mileage caps on specialty vehicles in some markets, then in April moved to include most types of vehicles in several markets, including Los Angeles and San Francisco. A reservationist at National’s LAX location said the mileage caps are set at 100-150 miles per day, with charges of 20-25 cents per mile beyond that--but some cars still have no limits, so “you might luck out.”

Reynolds travels anonymously at the newspaper’s expense, accepting no special discounts or subsidized trips. To reach him, write Travel Insider, Los Angeles Times, Times Mirror Square, Los Angeles 90053.