by By Tom Isler | February 01, 2009

Speeding toward a greener future, car rental companies are adding fuel-efficient vehicles to their fleets and offering carbon-offset programs to customers. Some even have programs geared to corporate clients who are looking to minimize the environmental impact of their business travel.

The greenest of the rental companies are Enterprise Rent-A-Car, National Car Rental and Alamo Rent A Car, all owned by the Taylor family of St. Louis. Combined, the three companies have the largest collection of cars averaging 28 miles per gallon or better, approximately 440,000 vehicles in a fleet of 1.1 million; 264,000 have earned the Environmental Protection Agency's SmartWay certification, a government rating based on emissions and fuel efficiency; and 4,000 are gas-electric hybrids.

The only catch is customers can book vehicles only by class (e.g., compact or midsize), and while they can express a preference for a fuel-efficient vehicle, there’s no guarantee that they'll get one. An Enterprise spokesperson says the companies now are exploring solutions to allow customers to reserve fuel-efficient cars.

In January, the three rental companies launched a carbon-offset program with TerraPass, allowing customers to pay an additional $1.25 per rental to fund projects that reduce the amount of carbon dioxide in the atmosphere. The Taylors will match consumer contributions up to $1 million, on top of a recent $50 million pledge to plant 50 million trees and a $25 million donation to create a renewable fuel research center in St. Louis.

Other companies have their own green offerings. Hertz customers can reserve cars by make and model, including vehicles in the Green Collection, which average 31 miles per gallon or better. Approximately 40 percent of cars in the company's regular fleet achieve an average of at least 34 miles per gallon, and 105,000 vehicles are SmartWay-certified. Hertz also recycles oil and car parts and has enacted other responsible policies, such as signing contracts only with auto parts suppliers that meet certain environmental standards.

Avis Rent A Car and Budget Rent A Car offer a consulting service for corporate clients to calculate their greenhouse gas emissions and identify ways to reduce their environmental impact. The Avis Budget Group will advise clients how to improve fuel efficiency, either through better driving habits or the use of electronic toll payment services or GPS units that give real-time traffic alerts, enabling drivers to avoid lengthy backups. Corporate clients also have the option to buy carbon-offset credits for their travel. Avis and Budget have 2,500 hybrids in a combined fleet of about 375,000 vehicles.

Dollar Thrifty Automotive Group, parent of Dollar Rent A Car and Thrifty Car Rental, has a small collection of Toyota Priuses as well as 9,600 "flex fuel" vehicles that can run on ethanol, available now at approximately 1,500 filling stations across the country.