All depends on how they do their engine / catalyst “regeneration.” Some engines were to be designed so they didn’t use the urea injection, but would then run at a higher exhaust gas temp to “burn out” the soot in the cat.
Explain to me why, to meet the emissions requirements you degrade fuel efficiency with EGR (which I’d get ride of in a heart beat) and the higher EGRs which means more fuel burned for a diesel since they throttle with the fuel.
All you need to know about this..
State and Federal Treasuries “Profit” More from Gasoline Sales than U.S. Oil Industry
http://www.taxfoundation.org/publications/show/1139.html