Quantifying the independent effects of vehicle weight and size on overall vehicle safety is necessary in order to assess the risks and benefits of vehicle weight reduction. This paper describes the results of one-stage and two-stage logistic regression analyses of the effects of passenger vehicle weight, wheelbase, track, and footprint on fatalities per accident, accidents per exposure (e.g., vehicle-miles-traveled), and fatalities per exposure using national and multi-state traffic accident and exposure databases. The analyses were accomplished in two phases. The first phase used 1995 though 2000 calendar year data for 1991 through 1999 model year vehicles. The second phase used 2002 through 2008 calendar year data for 2000 through 2007 model year vehicles. The overall fatalities per exposure results tend to confirm the one-stage results previously reported by NHTSA, however the new two-stage results provide additional insight into the independent effects of weight reduction and size reduction on vehicle crash involvement, crashworthiness and crash compatibility. The sensitivity of the results to various databases, variables, and assumptions was also investigated. The results from both phases indicated that the estimated overall effects of passenger vehicle weight reduction (while holding vehicle size constant) on fatalities are small in comparison to other variables and may not be statistically significant depending on various key assumptions (e.g., controlling for changes in wheelbase and track versus changes in footprint, and the type of induced-exposure data that are used), and this is due to small or opposing effects of these variables on crash involvement, crashworthiness and crash compatibility.