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In an effort to reduce nitrogen oxide (NOx) emissions and other pollutants from heavy-duty vehicles (HDVs), regulators have been implementing more stringent regulations that have included a combination of significantly more stringent emissions standards with the introduction of battery electric vehicles (BEVs). This study analyzed in-use NOx emissions data from 63 HDVs across various vocations, model years, and engine technologies/fuels to assess which current technologies offer a realistic path toward reducing NOx emissions without significantly burdening fleet operators or electrical infrastructure. All 63 HDVs were equipped with portable emissions measurement systems when they were tested for in-use NOx emissions during their routine operation on California roadways. The data was analyzed using the moving average window method proposed by the Environmental Protection Agency (EPA) in which the in-use emissions are broken up into two bins dependent on the engine load: ≤6 % (idle) and >6 % of maximum rated power. It was found that diesel engines manufactured after 2020 and natural gas engines certified to the 0.02 g/bhp-h NOx standard met the 2027 and 2035 EPA in-use NOx standards for both bins even though the future standards do not apply to these older engines. In addition, over an 80 % reduction in average NOx emissions is seen in both bins and fuels as modern NOx and greenhouse gas standards were implemented in 2017. With the implementation of ultralow NOx diesel technology engines, capable of meeting 0.035 g/bhp-h NOx limits, it was found that reductions in the NOx emissions inventories from 90.0 to 91.9 % could be achieved by 2050, depending on the deployment of BEVs. In conclusion, current and upcoming engine technologies can serve as benchmark powertrain solutions for emissions inventory reductions in the near and intermediate terms solutions even to the extent that the transition to battery electric HDVs becomes more gradual.