It's getting more difficult for business travelers to find the best car rental rate.

Using computer programs to monitor competitors' prices, as well as supply and demand, auto rental companies are more frequently changing prices, particularly at airports.

Prices can change many times during a day. They can rise 50 percent or more, or drop at least 20 percent, within 24 hours. In some cases, rental car companies make only a slight adjustment in their rates. It's a practice that's becoming increasingly common in the rental car industry and one not widely known even among experienced travelers.

"It's confusing out there for customers," says car rental consultant Mike Kane.

The average time between price changes has been decreasing since about 2003, he says.

Charles Pulley, a spokesman for the company that owns National and Alamo, says changes "may occur 10, 20 or more times a day" when there's heavy demand for vehicles.

Customers' increased use of the Internet to monitor prices and make reservations has caused car rental companies to adjust their rates more frequently. Competitors' prices are more transparent, and adjustments are needed to remain competitive.

A small price drop can result in more bookings, and a minor increase can bring in a lot of revenue, Kane says.

Rates often reflect demand

The companies vary from one another in their frequency of price changes.

During a five-day period this month, for example, the price of a compact car rented from Thrifty or Dollar at San Francisco's airport changed five times, according to spokesman Fred Fleischner. Thrifty and Dollar are owned by the same company.

Rates for a one-week rental for a car to be picked up today ranged from $107.25 to $166.90.

The company says in some instances it may change prices four times daily, or maybe more.

By contrast, rates at Hertz, the largest company at airports, change weekly at big airports and less frequently at smaller ones, Vice President Richard Broome says.

They could change daily "when unexpected, severe weather shuts down an airport, and we have a run on cars rented one-way, leaving us with no cars for the next day."

The prices for cars in highest demand change most often. Avis spokeswoman Susan McGowan says the prices for compact, intermediate and full-size cars change more often than other types of vehicles. Avis and Budget, a related brand, may change "multiple times" a day.

At National, the rates of midsize vehicles may change the most, Pulley says. At Thrifty, Dollar and Enterprise, prices change most frequently in five types of vehicles, including economy and full-size cars.

Rates for rentals from sites away from airports tend to change less frequently. Darren Arrington, director of revenue management for Thrifty and Dollar, said competition in that part of the business is less direct than at airports. Also, he says, there's less advance demand at those sites.

At Enterprise, the industry's largest off-airport company, rate changes average "once a day or less," spokeswoman Christine Conrad says.

It pays to shop around

Frequent business traveler Sue Reiss, of Marathon, Fla., says she was unaware that auto rental companies change their prices various times during a day.

"I find that pretty shocking," says the regional manager of a transportation safety company. "If the cars are available, why would there be a reason to vary those prices? If the car is worth $50 per day at 9AM, it should have the same value at noon."

Frequent renter Frank Stasiowski, of Newton, Mass., says a policy of frequent rate changes "stinks, but what can a consumer do?"

He says he always questions his rate when picking up a car. "About 50 percent of the time the manager lowers my rate somehow magically," says the CEO of an engineering company. "I guess most customers just never ask."

Renters who aren't as fortunate as Stasiowski may want to shop around for lower rates. There can be substantial price differences between companies.

On March 6, for example, Enterprise charged $26.88, including taxes, on its Web site for a Wednesday pickup of an economy car -- a Chevrolet Aero -- at the Los Angeles airport. Hertz charged $72.59 for its economy car -- a Hyundai Accent.

A full-size Ford Taurus costs $78.88 at Enterprise or $103.36 at Hertz, while a full-size Dodge Charger costs $35.84 at Thrifty.

Auto rental companies say it often pays to book in advance. "The price usually, but not always, goes up as you get closer to the arrival date and time," Fleischner says.

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