Small, light vehicles consume less fuel to go the same distances as larger, heavier vehicles. At the same time the light ones afford much less protection in crashes. This is why it's important to find ways to conserve fuel without promoting small, lightweight vehicles. Any regulatory action that increases sales of such vehicles will increase injury risk in crashes.
The graph below shows relationships between vehicle weight and driver death rates (deaths per million registered vehicles) and fuel consumption (gallons per 100 miles). The highest death rates and lowest fuel consumption are for the lightest passenger vehicles. Heavier vehicles have lower death rates and greater fuel consumption.
However, the safety benefits of additional vehicle weight (to the occupants of those vehicles) diminish as vehicles get heavier and heavier. Still their fuel consumption continues to increase. The optimum fleet mix to enhance safety would eliminate both the lightest and heaviest passenger vehicles. By far the bigger benefit would come from eliminating the lightest ones, which are hazardous to their occupants in all kinds of crashes. Eliminating the heaviest ones would enhance safety in crashes between two passenger vehicles, but the overall safety effect would be much smaller. In considering how to improve fuel economy, it makes sense to look for ways that don't promote lightweight vehicles and that would encourage downweighting of only the heaviest vehicles (those weighing about 4,000 pounds or more).
Relationships between vehicle weight and driver death rates and fuel consumption
Note: Relationships between death rates and vehicle weights reflect fatal crashes of 1995-99 models during calendar years 1996-2000. The rates were adjusted to account for some differences in driver age and sex within and between vehicle types. Remaining differences in vehicle use patterns and driver demographics may account for some of the death rate differences within and between vehicle types.