However, none of the withdrawals pertained to the use of GEM or the new version of GEM with revised rounding.3

The cooperative agreement between NHTSA and the NRC, modified in September 2013, includes the following element of the amended statement of task (Appendix B) that is of relevance to this chapter:

The committee will analyze and provide options for improvements to the certification and compliance procedures for medium- and heavy-duty vehicles—including the use of representative test cycles and simulation using various models—such as might be implemented in revised fuel consumption regulations affecting MY 2019-2022.

The committee has talked with several but not all vehicle manufacturers that use GEM. Based on those conversations, on its review of GEM, and on the need to acknowledge efficient technology broadly in a simulation model, the committee makes several recommendations in the present report. Execution of GEM requires insertion of data on aerodynamic properties and tire rolling resistance. These data are obtained from measurements, which are also discussed below.


The GEM model was developed for NHTSA’s and EPA’s Phase I Rule on medium- and heavy-duty vehicles (MHDVs) as a simplified method for determining the effects of the vehicle (rather than the engine) on fuel economy and GHG emissions. There are separate regulations focused on a certification of the engine as meeting established criteria for carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). As such, it builds upon the work already done by EPA and the established procedures in 40 CFR Part 1065 to certify engines in a test cell for the criteria pollutants carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons, and particulate matter (see Box 3-2). The executable GEM program is based on MATLAB/Simulink,4 a common language for modeling and simulation in engineering. The user has five main inputs to the model:

  • Aerodynamic drag coefficient value by bin,
  • Steer and drive tire rolling resistance by value,
  • Vehicle speed limiter presence and value,
  • Vehicle mass reduction (lb), and
  • Idle reduction presence.

Additional opportunities are classified as Innovative Technologies, which require a separate procedure and submittal process for taking advantage of the credits allowed. Hybrid technology is included in the Advanced Technology and Innovative Technologies categories. Natural-gas-powered vehicles, which will replace a fraction of the diesel-powered vehicles and which will be important in the coming years, are also included in the Advanced Technology and Innovative Technologies categories. Chapter 4 of the Regulatory Impact Analysis (RIA) that accompanied the promulgation of the Phase I Rule (EPA and NHTSA, 2011) provides an extended explanation of the model.

BOX 3-2
Criteria Pollutants

The standards for criteria pollutants are found at Title 40 Code of Federal Chapter 1037.102 entitled “Exhaust emission standards for NOx, HC, PM, and CO.” These pollutants are sometimes described collectively as “criteria pollutants” because they are either criteria pollutants under the Clean Air Act or precursors to the criteria pollutant ozone. These pollutants are also sometimes described collectively as “non-greenhouse gas pollutants,” although they do not necessarily have negligible global warming potential. As described in § 1037.102, standards for these pollutants are provided in 40 CFR part 86.

A brief overview is obtained by looking at the input screen of the model, shown in Figure 3-1. The output of GEM is cycle-weighted g/ton-mi CO2 and gal/1,000 ton-mi. An important part of understanding the model consists in examining the underlying assumptions, including the following:

  • Fixed engine fuel map for one fuel type, diesel;
  • Fixed manual transmission (10-speed for Classes 7 and 8 and 6-speed for vocational);
  • Fixed axle ratios;
  • Fixed tire sizes and a rolling resistance that is invariant with respect to speed and torque, though user supplied;
  • Fixed electrical load;
  • Fixed mechanical accessory power;
  • Three fixed cycles (California Air Resources Board [ARB] Transient, 55 mph Cruise, 65 mph Cruise);
  • Level roads;
  • 1,800 hours engine idle;
  • A fixed payload, although augmented with a weight reduction option; and
  • A value for the product of area and drag coefficient, which is a user-supplied constant.

Since GEM is a relatively simple model focused on aerodynamics, rolling resistance, speed, weight, and idle control, it is not capable of acknowledging efficiency or


3 This change was “to address rounding inconsistencies when converting CO2 values to equivalent fuel consumption values in the Greenhouse Gas Emissions Model (GEM) simulation tool” (76 Fed. Reg. 36377).

4 For further information, see Accessed November 14, 2013.

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