Image credit:USDOE, Emissions of Greenhouse Gases Emissions of Greenhouse Gases in the United States, 2008 (pdf file)
Nitrous oxide, which has 298 times more climate forcing impact per unit weight than CO2, is an invisible gas emitted by the millions of tons from combustion sources (see graphic). The internal combustion engine, as indicated in the figure, is the primary source of combustion-associated nitrous oxide (N2O) - see update below for explanation of 'combustion associated.' Note that vehicular N2O emissions are more related to miles traveled, and less related to fuel efficiency (mpg or Km/liter). Therefore, buying a more fuel efficient internal combustion powered vehicle makes little positive difference with N2O emissions.
Update: it appears that some readers are confused about why N2O emissions are not proportional to fuel efficiency. Here is a short explanation, for those who are interested.
NOX gases (N2O is one of several chemical species that make up the group called "NOX") are formed when atmospheric nitrogen and oxygen are united at the high temperature of the combustion chamber. Hence, NOX formation is driven by cylinder temperature mainly, and also by air intake rate. Because there can be a rough proportionality between air intake and fuel efficiency, depending on engine design and size, I used the phrase "little positive difference."
Hybrid vehicles do emit less N2O per vehicle mile traveled.
The least N2O-emitting vehicle would be all electric, charged via grid fed power largely from nuclear and/or renewable sources. (An electric vehicle charged by a largely coal-powered grid would still emit less N2O than an ICE powered one.)
Note: I purposefully have restricted this post to comparing propulsion system designs. Fertilizers and cattle manure are, of course, very large N2O emission sources, world wide. But, in general, we humans don't eat our cars or drive cattle (except in John Wayne movies).
More nitrous oxide posts.
Nitrous Oxide Emissions from Ocean Bacteria on the Rise
But what about Laughing Gas Emissions?
Genetically Modified Rice to Fight Global Warming in China ....
Study Finds Meat and Dairy Create More Emissions Than Miles ...