http://www.insightcentral.net/KB/compar ... e-tdi.html
High MPG = low CO2 (carbon dioxide) emissions
NOx (nitrous oxides), CO (carbon monoxides), HC (unburnt hyrdrocarbons) are other emissions, and they are not dependant on the MPG of a vehicle, but the setup of the engine and emissions systems. (I think O3 (ozone) is in there too, but I'm too lazy to look right now.)
For example, there are the PZEV versions of the Ford Focus, Nissan Sentra, Toyota Camry, and a few others, that match the emissions levels of the AT-PZEV versions of the Honda Civic Hybrid and the 2004 Prius (the latter 2 with significantly higher MPG than the former).
Depends if you're only concerned with global warming (CO2) emissions, or if you're also concerned about local low-level air pollution (smog, acid rain, etc.)