California Code, Health and Safety Code - HSC § 44251

(a) The state board shall specify smog index numbers for new light-duty passenger vehicles and light-duty trucks with a gross vehicle weight up to 6,000 pounds to be sold in California.For gasoline and alternative fuel vehicles, that smog index shall be based on certification data quantifying tailpipe and evaporative emissions of ozone precursor chemicals for classes of vehicles.

(b) For diesel fuel vehicles, the smog index shall be based on certification data quantifying tailpipe emissions of ozone precursor chemicals and particulate matter. Particulate emissions from diesel fuel vehicles certified to model year standards that did not include a particulate limit may be assumed to be equal to particulate emissions for model year 1985 diesel fuel vehicles.

(c) The state board shall specify the relative weight of emissions of ozone precursor chemicals and particulates in the smog index values for diesel vehicles. This weighting shall be based on the relative importance of each category of emissions to air quality problems in California.

(d) Smog index number 1.0 shall be assigned to a hypothetical light-duty passenger vehicle, a hypothetical light-duty truck with a gross vehicle weight of 3,750 pounds or less, and a hypothetical light-duty truck with a gross vehicle weight of greater than 3,750 pounds up to 6,000 pounds, emitting the maximum amount of pollution allowed for that class of vehicle certified for sale in this state as of the January 1 immediately preceding the operative date of this section. The state board shall determine the existing class or classes of vehicles to which the smog index shall be applied.