With the increased use of particle traps and nitrogen oxide (NOx) control devices to reduce air pollution, modern diesel vehicles are being encouraged over gasoline vehicles globally as a central method of slowing global warming. Data to date, though, suggest that the NO2:NO ratio from modern diesel may exceed that of gasoline, and it is difficult to reduce diesel NOx below gasoline NOx without increasing particle emissions. Here, it is calculated that, unless the diesel NO2:NO ratio and total NOx are reduced to those of gasoline, modern diesel, which should have lower hydrocarbon (HC) and carbon monoxide (CO) emissions than gasoline, may still enhance photochemical smog at the surface and aloft, on average, over the U.S. relative to gasoline. The reason is that vehicle-produced smog in the U.S. depends more on NOx and the NO2:NO ratio than on HCs or CO. It is also found that vehicle NOx controls may be more effective than NO2:NO ratio controls at reducing ozone.