This Technicians guide details the changes that have been made in the gas formulation process as well as numerous other pertinent informational topics for all interested parties. The auto manufacturers have made tremendous strides in reducing vehicle emissions, which was characterized by the introduction of the TIER I vehicles in 1999 which substantially decreased the exhaust emission levels. This, however, is not sufficient to adequately clean the air in some regions of the country, thus the program of reformulated gasoline was initiated in 1987. Two significant changes have been in the gasoline formulation process since.