Emission measurements from heavy-duty vehicles have been collected using two different measurement techniques. The On-road Heavy-duty Monitoring Systems (OHMS) or using our FEAT optical remote exhaust sensor. Both techniques measure tailpipe emissions as a ratio to emitted carbon dioxide, and thus fuel specific emissions. However, the OHMS approach is capable of collecting superior particle emission measurements (particle mass, particle number and black carbon mass), while the FEAT system collects a larger suite of gaseous emissions.