Instead of looking at NOx (Nitrous Oxide) emissions, we should be looking at overall pollution of a vehicle from the point of manufacture until the end of its serviceable life.
What if the CAFE standard were based upon:
1. Pollution to produce entire vehicle. Engine, parts, components, etc. Calculated at each factory from those that make seats and dashboards, to electronic components, to the factory that finally assembles the car.
2. Serviceable life span (years and miles)
3. Fuel usage (MPG) (miles per charge, etc) and Cost to obtain that fuel.
4. Impact that would occur if that vehicle weren't disposed of properly ( i.e. totaled vehicle spilling it's fuel at the point of an accident).
Battery powered vehicles would have a strong position in terms of cleanliness at their "tailpipe", but would suffer in terms of ecological impact of damaged, improperly destroyed battery packs, and cost/issues of production.
Then the "Overall Impact" wouldn't just concentrate on tailpipe pollution as an end-all/be-all, it would include the overall impact of that vehicle. Including gasoline engined vehicles which tend average half the usable life of a diesel vehicle. So- in essence, generally speaking Gasoline cars must be produced about twice as often as a diesel.