In 2015 the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Transportation's National Highway Traffic Safety Administration (NHTSA) proposed a new steady-state engine dynamometer test procedure by which heavy-duty engine manufacturers would be required to create engine fuel rate versus engine speed and torque “maps”. These maps would then be used within the agencies' Greenhouse Gas Emission Model (GEM) for full vehicle certification to the agencies' proposed heavy-duty fuel efficiency and greenhouse gas (GHG) emissions standards.This paper presents an alternative to the agencies' proposal, where an engine is tested over the same duty cycles simulated in GEM. This paper explains how a range of vehicle configurations could be specified for GEM to generate engine duty cycles that would then be used for engine testing. This paper discusses a numerical scheme by which GEM could interpolate these cycle average results instead of the steady-state map the agencies proposed. This paper explores this alternative via simulation and numerical analysis. Engine and powertrain testing of this alternative are described a companion SAE Technical Paper entitled “Alternative Engine Mapping Approach for Simulation of Vehicle CO2 Emissions.”These two papers demonstrate how this alternative could successfully integrate cycle average engine test results into GEM for full vehicle certification. Advantages include capturing measured transient engine performance within GEM and avoiding disclosure of potentially proprietary engine control strategies, which could be revealed by the agencies' proposed procedure. This alternative might also lead to a more robust process for de-normalizing test cycles for engine certification.