Light-duty vehicles account for about 20 percent of US CO2 emissions. However, new vehicle fuel economy standards have not been significantly tightened since they were first enacted three decades ago. A historical impediment to imposing tougher fuel economy standards has been the long-standing perception that reducing the mass of a car or truck would make it more dangerous to its occupants in a crash. One often hears that this perception is dictated by "simple physics": that, all else being equal, you are at greater risk in a lighter vehicle than in a heavier one. Our research on driver fatality risk has found that, when it comes to vehicle safety, all else is never equal. Vehicle mass is not the most important variable in determining occupant safety, not even in frontal crashes between two vehicles. You are at no greater risk driving an average car than you are driving a much heavier (and less fuel efficient) SUV. And larger and heavier SUVs and pickups impose enormous risks on car occupants. This talk will summarize several years of research on the interplay between vehicle weight, size and safety, and what the implications are for new state and federal standards to reduce vehicle CO2 emissions.