At Edison2 we pursue low weight as the first of two virtues to maximize efficiency in the automobile; the second is low aerodynamic drag.
One economy that accompanies a move to low weight is in the cost of materials. Low weight means not only less materials used, and hence lower production costs, but also lower maintenance costs, such as for brakes and tires (and roads).
The greatest savings with low weight is in energy consumption while driving. These savings are a function of the aerodynamic and resistance drag and the actual weight of the vehicle. Other factors include type of road, drive cycle (stop-and-go vs. highway) and vehicle features, which can be negative (air conditioning) or potentially beneficial (regenerative braking).
A heavier car can regenerate more energy with braking than a light car, and of course the more often brakes are used the more chances there are to recapture energy. Hybrid systems are most effective for buses and delivery trucks, which are heavy and stop frequently. On the other hand, if a car is used with very few stops the regeneration system is only added weight.
Conventional cars have become really heavy. Partly this is due to the weight of equipment needed to meet emissions and safety requirements, partly it is due to feature creep – the weight of motors and brackets for fully adjustable seats, for example – and partly because weight begets weight. The size and stress put upon each component of a car is a function of the total weight of a car; this is obvious in the need for a more powerful engine or larger brakes in a heavier car but the concept applies to many other vehicle parts.
Therefore is harder to take weight out of a car than to design a new one; the Very Light Car is light because it is light.