Enterprise Rent-A-Car is leading the way in green initiatives for rental companies. It has the largest fleet of fuel-efficient cars, including 440,000 that get 28 mpg, 5,000 hybrids and 73,000 flex-fuel cars. Not to mention, Enterprise recently began offering a carbon-offsetting fee for its customers.

But, there are some hiccups in its green efforts. The flex-fuel vehicles are rarely filled with E85 due to the lack of available fueling stations (only an estimated 1,400 nationwide). Less than 10 percent of its customers are opting for the $1.25 emission-offsetting fee. And, hybrid automakers don’t have enough supply to match Enterprise’s demand.

(More details after the jump)

Enterprise’s chief executive Andy Taylor, in an interview with CNN Money, said, “We’ve told our manufacturing partners that we want many, many more. You just can't get them.