The workhorse of taxi fleets across the U.S. is the Ford Crown Victoria. Also popular as a cop car, the Crown Vic is known for its body-on-frame design (easy to repair) and impressive cargo and interior space. One area it does not excel in is gas mileage, as the big V8 engine aren't all that efficient to begin with. Task them with the stop-and-go nature of city driving, and gas mileage plummets.
New York City has been trying for several years to force taxi owners to switch to hybrids. A recent ruling by a Federal judge backed up an earlier ruling that barred the city from mandating hybrid taxis.