Compared to parallel hybrids like the General Motors Two Mode system, the series hybrid eliminates the weight and complexity issues of a transmission. For an application like a bus, the added unsprung mass of the wheel motors doesn't really impact the ride and handling the way it would in a car. Parallel hybrid buses boost mileage by about 25-30 percent while a series hybrid can push that to 50 percent.
Efficiency is boosted further by having a GPS system input that shuts down the generator and lowers the power when operating in congested areas where less speed is needed.
[Source: Technology Review]