One distinct advantage of electric ground vehicles is that they can reduce their total energy consumption by converting forward kinetic energy back into stored electrical energy: a technique known as regenerative braking. This makes them draw less power per kilometre travelled (especially in stop-start city traffic) and increases the effective range of any particular battery pack. Some existing vehicles reduce their energy consumption by about 15% using this technology.
While flywheels could do something similar in vehicles powered by other means (such as biofuels), regenerative braking doesn’t require much extra hardware. Also, if electric vehicles end up using one motor per wheel, traction control capabilities could be easily incorporated.
A low-tech approach with similar properties is used in some subway systems. Stations are built at a higher level than tracks. As trains leave, they run downhill and gain speed as gravitational potential energy is converted into kinetic energy. As they approach the next station, they go uphill and make the conversion in reverse.