The finding that the relative safety disadvantage of small compared to large cars for post-1980 cars is less than for pre-1980 cars has stimulated speculation that increasing Corporate Average Fuel Economy (CAFE) standards would increase fatalities less than previously expected. This study derives relationships between fatality risk and car mass as a function of car model year in order to better estimate future car size effects on safety. Relative driver fatality risk versus car mass is examined using Fatal Accident Reporting System data for crashes between two cars of specific model year. The results can be expressed in terms of r, the ratio of driver fatality risk in the lighter car compared to that in the heavier car when the heavier car has a mass 50% greater than that of the lighter car. For 1966-1979 model year cars r varies between 3.7 and 5.1;​ for 1984 cars r = 2.4;​ and for 1989 cars r = 3.8. All cars having airbags increase the ratio of risk in small compared to in large cars. The results suggest that the lesser mass effect observed for mid-1980s cars was because improved crashworthiness features appeared in small cars earlier than in large cars. As all cars become redesigned, the relationship between risk and mass will approach that observed earlier. Future fatality increases from CAFE increases will be greater than estimated using mid-1980 data.