Although the fleets of the nation's eight major rental car brands include many  vehicles that are highly rated for protecting against head-on collisions, a USA Today analysis of the fleets finds that many of the vehicles aren't rated nearly as safe if motorists are hit at the side or from behind or are involved in rollover accidents.

The analysis used Insurance Institute for Highway Safety ratings to study the fleets.

The analysis reveals that although more than 95 percent of 167 different vehicles in rental fleets are rated "good" in head-on crashes, a large disparity exists in their safety ratings in side, rear and rollover crashes.

According to USA Today's matching of 213 types of vehicles in the rental car fleets with IIHS crash data for model-year 2011 vehicles, 45 vehicles had no ratings for front, side or rear crash tests, and 105 were not tested for roof strength, an important factor in rollover accidents.

Crash ratings aren't the only factor to consider in rental car safety. Maintenance, safety equipment in each vehicle and driver performance in an often unfamiliar vehicle also come into play.

In August, two auto-safety advocacy groups-the Center for Auto Safety and Consumers for Auto Reliability and Safety-petitioned the Federal Trade Commission to stop Enterprise Rent A Car and its parent company, Enterprise Holdings, from renting recalled-but-unrepaired vehicles.

A spokeswoman for Enterprise Holdings, which also owns the Alamo and National brands, says that the company has improved and strengthened its approach to recalls over the past decade.