The Environmental Protection Agency (EPA) has adopted Tier 2 Standards for Vehicle Emissions for application on passenger cars and light-duty trucks of 8,500 lbs. GVWR or less as well as for medium-duty passenger vehicles between 8,501 and 10,000 lbs. during the model years 2004 to 2007. Tier 2 represents the increasingly stringent regulation of nitrogen oxides (NOx) and particulate matter (PM). Medium-size passenger cars capable of carrying more than 12 passengers, however, are considered to belong to the light-heavy-duty truck category, and are not subject to Tier 2 regulation.

Table 3: Future Emission Standards for Passenger Cars and Light-Duty Trucks (In detail, vehicles are divided into 11 classes, and regulated per the average value for each combination)

From DOE Report NREL/SR-540-32689

Vehicles are divided into 11 different classes, and a manufacturer's fleet average NOx value is calculated with the stipulated NOx value for each class of vehicle sold by the manufacturer, using a weighted average based on sales volumes to determine compliance with the figures shown in Table 3 for NOx, PM, and Non-Methane Organic Gas (NMOG). This strengthening of Tier 2 emission standards does not take into consideration differences in car weight or whether a vehicle is gasoline- or diesel-powered, and is the same across the board for all categories of vehicles. Tier 2 compliance, however, is phased in across several model years.

Table 4: Classes of Vehicles Required to Comply with Tier 2 Standards

Applicable to passenger cars and light-duty trucks with a GVWR of 8,500 lbs. or less as well as medium-duty passenger vehicles with a GVWR of between 8,501 and 10,000 lbs. and carrying 12 or fewer passengers.

From DOE Report NREL/SR-540-32689

Compliance with Tier 2 Standards is mandatory for all passenger cars and light-duty trucks beginning with 2007 model year vehicles. Some medium-size passenger cars carrying 12 passengers or less will be added beginning with the 2008 model year, and beginning with the 2009 model year, all medium-size passenger cars carrying 12 or fewer passengers.