Relative risk of driver death in a two-car crash is estimated as a function of the mass of each car using driver and (to measure exposure) pedestrian fatalities coded in the Fatal Accident Reporting System (PARS). Because mass effects are less strong for more recent cars, the study uses model-year 1980 and later cars. A three parameter analytical function fitted to the estimated risks generated the following findings. When a driver transfers to a lighter car that driver’s fatality risk increases, while the other involved driver’s risk declines. When cars of the same mass crash into each other, fatality risk is lower when both cars are heavier. If one of the equal-mass cars is replaced by another lighter by any amount, the increase in fatality risk to the driver in the car of reduced mass exceeds the reduction in fatality risk for the driver in the unchanged car (that is, net risk increases). Net driver fatality risk (or net fatalities) in a car population increases if any car in the population is replaced by a lighter one, or if one population of identical cars is replaced by another population of lighter identical cars. Making all cars in a population of uniform mass while keeping the average mass constant increases net fatality risk. The safety disadvantage of lighter and smaller cars is intrinsic—it cannot be negated by design changes if these are also available to larger heavier cars because, when all other factors are equal, more mass and more space can be used to provide increased safety. Fuel economy and safety (like higher speed limits and safety) are intrinsically in conflict;​ individual and national choices require a balancing of advantages.