Study: Excessive Real-World NOx Emissions from HD Diesel Pickups
TRUE Initiative data shows emissions increase as vehicles age, but questions remain.

An eight-year study of in-use emissions from heavy-duty diesel pickups in Colorado and Virginia showed those vehicles to be emitting oxides of nitrogen (NOx) far in excess of regulated maximums. Although all the diesel-engine pickups demonstrated excess emissions, the Ford F-250/350 from 2010 to 2019 model years consistently was the highest emitter of the group that included the Ram 2500/3500, Chevrolet Silverado HD and GMC Sierra HD, with NOx emissions from three to 10 times more than permitted by light-duty vehicle (LDV) emissions regulations adopted for heavy-duty pickups in 2016.

The study, which collected emissions data between 2015 and 2023, was conducted by The Real Urban Emissions (TRUE) Initiative, an independent nonprofit advocacy group that is a partnership of the FIA Foundation and the International Council on Clean Transportation (ICCT). More than 360,000 measurements were collected, the study authors said, and each model was measured at least 70,000 times with a minimum of 10,000 unique vehicles for each model. The roadside remote-sensing equipment used was the Opus RSD 5000 series, said Michelle Meyer, researcher at ICCT and one of the study’s two authors.
“High real-world emissions exceeding laboratory limits may have several causes. Nearing ten years old, these vehicles are subject to routine deterioration. Individual vehicle malfunctions, tampering with emission control systems, and auxiliary emission control device (AECD) could be other causes,” the TRUE Initiative said in a release. “When disclosed and approved by authorities, AECDs are legal, whereas undisclosed and unapproved AECDs are known as ‘defeat devices.’”
Hardware deterioration is a particular aspect to consider due to the increasingly sophisticated hardware required to bring heavy-duty diesels into compliance with ever-stricter emissions standards. Escalating fuel-injection pressures stress fuel-system durability, while rising cost and complexity of exhaust aftertreatment systems also complicate the robustness of long-term emissions performance. Meanwhile, as was demonstrated by the Volkswagen “Dieselgate” situation and the more recent U.S. Dept. of Justice settlement with Cummins to recall some 600,000 Ram 2500/3500 models fitted with the Cummins-made diesel engine, software can be a controlling factor in a diesel engines’ emissions “profile.”
“Tampering remains an important issue to address in parallel with manufacturer-level high-emitting vehicle models,” Meyer said. “Different forms of vehicle testing, inspection and maintenance programs and market surveillance can help analyze and address each of these issues.”
Ford emits most
The study showed that Ford heavy-duty pickups persistently produced more NOx emissions than do other diesel-engine pickups, even the Ram models employing the recalled Cummins engines. Model-year 2018 and 2019 Ford trucks had approximately twice the NOx emissions of the next highest emitter, the Ram 2500/3500, the study noted.
“Unlike the other three models, MY 2016-2019 Ford trucks, certified to interim [emissions regulation] bins, were not required to certify to the Supplemental Federal Test Procedures (SFTP),” the study noted, adding that “[model year] 2020 Ford trucks, which were certified to the SFTP, showed a substantial decrease in emissions that brought them in line with the other three models.”

The full case study reporting the findings can be found here .
“To our knowledge, the chassis-certified Ford diesel trucks use an unconventional layout of emission control systems, with the SCR placed before the DPF. Most other truck models (and engine-certified Ford trucks) are designed with the SCR placed after the DPF,” Meyer told SAE Media. “However, we are unsure if this could be a reason for the high emissions observed from Ford trucks in this analysis.” A Ford representative did not respond to a request for comment.
Pickups of each brand showed relatively linear increases in NOx emissions as vehicles age. But the eight-year study also found that higher engine loads (determined as a function of vehicle specific power, or VSP, a proxy for engine load) also resulted in outsized NOx output, particularly for the Ford and Ram pickups. And temperature was a factor for the Ford engines: “2010-2015 Ford F-250/F-350s showed temperature dependencies and emitted nearly two times higher NOx at low temperatures than at high temperatures. This trend was not observed for other vehicle models,” the study noted.
“Addressing the high real-world emissions from these vehicles that will continue to emit high NOx is important for improving air pollution and public health,” Meyer stressed. “This analysis showed that in just one year of operation, [model year] 2010-2019 Ford diesel trucks contribute roughly 10,800 to 28,800 metric tons of excess NOx emissions.”
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