The National Renewable Energy Laboratory (NREL) contracted with conversion companies in six states to convert approximately 900 light-duty Federal fleet vehicles to operate on compressed natural gas (CNG) or liquefied petroleum gas (LPG). Aftermarket conversions can play an important role in the transition to the more widespread use of alternative fuel vehicles. In this program, they have been successful in helping the Federal government meet the vehicle acquisition requirements of EPACT, establishing infrastructure, and increasing the displacement of imported oil. The disappointing emissions performance to date of these closed-loop feedback kits, however, raises the question of their overall emissions contribution to the environment. This is especially important when considering converting the latest model year vehicles, which have been certified to the lower (Tier 1) emissions standards. 19p. establishing infrastructure, and increasing the displacement of imported oil. The disappointing emissions performance to date of these closed-loop feedback kits, however, raises the question of their overall emissions contribution to the environment. This is especially important when considering converting the latest model year vehicles, which have been certified to the lower (Tier 1) emissions standards. 19p.