Objectives:​ Proposed increases in corporate average fuel economy standards would probably lead to lighter cars. Well-established relationships between occupant risk and car mass predict consequent additional casualties. However, if size, not mass, is the causative factor in these relationships, then decreasing car mass need not increase risk. This study examines whether mass or size is the causative factor.

Methods:​ Data from the Fatal Accident Reporting System are used to explore relationships between car mass, car size (as represented by wheelbase), and driver fatality risk in two-car crashes.

Results:​ When cars of identical (or similar) wheelbase but different mass crash into each other, driver fatality risk depends strongly on mass;​ the relationship is quantitatively similar to that found in studies that ignore wheelbase. On the other hand, when cars of similar mass but different wheelbase crash into each other, the data reveal no dependence of driver fatality risk on wheelbase.

Conclusions:​ Mass is the dominant causative factor in relationships between driver risk and car size in two-car crashes, with size, as such, playing at most a secondary role. Reducing car mass increases occupant risk.