In hopes of attracting air travelers who might be thinking twice about shelling out for gas on top of rental-car rates and fees, agencies like Avis, Enterprise and Hertz are offering special deals in which rates are cut by 50 percent.

With an advance reservation, one can get a full-size car such as a Chevrolet Impala at the Los Angeles airport Hertz pickup for a total of $49.50 on a Friday and return it by Monday. The discount applies to the weekend rate before taxes, fees and airport charges, which raises the total rental to $74.82, with unlimited mileage. The normal rate is $124.32.

Other companies, like Alamo and National, are discounting certain options, such as navigation systems with turn-by-turn audio directions and routes to millions of locations, including gas stations and restaurants. Instead of paying the normal daily rate of $11.95, renters are getting the navigation systems for $5.98.

Unfortunately, many of the deals rental agencies are offering are set to expire at the end of June and carry a variety of restrictions. It's unclear if the deals will extend into the peak travel months of July and August. Neil Abrams, president and founder of Abrams Consulting Group in New York, says deals like these are "relatively new." Also, most of the deals don't apply to drivers age 25 and younger, and a 24-hour advance reservation is required.

Visitors to Hawaii and the New York City metro area don't qualify for Hertz's 50 percent off weekend rentals that apply to small economy cars and larger full-size vehicles. And to save $30 on National's weekly car rental with unlimited miles, you have to keep the vehicle over a Saturday night and rent it for a minimum of five days.

Furthermore, many of the troubles plaguing the auto industry are showing up in the car-rental fleets, like slow-moving, gas-guzzling sport utility vehicles. That's why Alamo is renting mid-size SUVs at lower rates than smaller, compact SUVs. Vacationers can rent a mid-size SUV, like the Chevrolet Equinox, with an Environmental Protection Agency combined estimated average of 19 miles per gallon, for $37.99 a day, or $20 to $25 less than a compact SUV, like the Kia Sportage (21 mpg) or Chevrolet Uplander minivan (19 mpg).

Only a handful of car-rental agencies, like Hertz, even have fuel-efficient or environmentally friendly "green cars" in their fleet, like hybrids and fuel-sipping cars. Hertz is shaving off $25 on weekly rental rates in hopes of drawing attention to this newly added fleet.

The good news is that some deals don't expire. If you reserve your vehicle with Alamo on its Web site and pre-pay with a credit card, you will automatically receive a 10 percent discount on the rental.

Dollar and Thrifty have an ongoing regional deal in the Dallas, Houston and Denver areas, as well as in the state of Florida. In the Texas markets you can get the Pass24 prepaid toll card for $8.95 per day, which covers all incurred toll charges in the eligible areas. In Florida, it costs $5.95 per day or $27.95 for the week.