I've noticed that the manufacturers tend to cover the rear and/or front wheels on the hybrid models more often than the regular vehicles. Are there practical or functional reasons for the covers? Is there something different about a hybrid that means a hybrid would get the covers, but a conventional car wouldn't?
The wheel covers, or fender skirts Wikipedia says they are commonly called, are fairings installed to reduce drag in the wheel wells and thus to increase range. According to the same Wikipedia article, the Honda Insight is the only current production car that uses them.
It is not so obvious, but I would expect to see that those cars would also have "smoother than average" underbellies as well.
The downside of using a fairings at the wheel wells is that it makes them harder to service, inspect, and change the tires. In cold and wet conditions they may also contribute to the build up of snow and ice in the wheel wells.
The same article also notes the use of fender skirts on city buses as a safety measure.