NOx exhaust aftertreatment systems are standard, so a compliant vehicle today will have considerably less NOx in exhaust as well. As for CO2, well, it's not a small chunk, but cars and light trucks account for less than half of transportation sector CO2 emissions; and the transport sector accounts for about 28% of overall CO2 emissions. It's not silly to reduce CO2 emissions in light trucks and cars, but at most reducing those sources would only cut about 15% of U.S. CO2.
Keep in mind, this is in a country where light trucks and SUVs account for a huge proportion of personal vehicles, and where the average age of a car on the road is about twelve years. As the window of vehicle years shifts further, you can expect that proportion to shrink.
