GHGs attributed to CNG buses are a result of in-use combustion and those for electric buses are emitted during the production of the electricity used for charging. For the GHG comparison in this study both EB and CNG buses are assumed to log 20,000 miles per year, based on 2014 Unitrans data [7]. The CNG buses are assumed to have a fuel economy of 0.4 therms per mile and emit 5.0 × 10−3 metric tons CO2e per therm where CO2, CH4, and N2O emissions are converted into CO2e using USEPA conversion factors [1] [7]. The typical EB (XE40) is assumed to have a fuel economy of 1.8 kWh per mile (see Section 2). The emissions factors for both the XE40 and CNG bus for each energy source considered in this study are summarized in Table 2.