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A heavy-duty engine is type-approved in engine dynamometers, while its in-service conformity is controlled on the road. In the first case, laboratory particle number systems (LABS) sample from a full dilution tunnel or a proportional partial flow dilution system (PFDS). In the second case portable emissions measurements systems (PEMS) measure directly from the tailpipe. Permitting in the regulation LABS sampling directly from the tailpipe would simplify testing and would improve their comparability with PEMS. In this study PEMS and LABS, both sampling from the tailpipe and measuring solid particles >10 nm, were compared with references systems (i.e. LABS from PFDS). One compressed natural gas (CNG) engine, and three diesel engines, all Euro VI step E, with or without urea injection, and with or without crankcase ventilation connected to the tailpipe, challenged the systems with different emission levels and particle sizes and properties. The results showed that the differences of the LABS to the references were in most cases within ±25%, with a few exceptions. The PEMS were within ±50%. There was no or small effect on the differences from engine technology, urea injection or crankcase ventilation. The inclusion of sub-23 nm particles increased 100% to 250% the particle number emissions. The urea injection increased the >10 nm emissions 300–600% (2–5 × 1010 p/kWh). Connecting the crankcase ventilation to the tailpipe further increased the >10 nm particle number emissions 340–560% (1.4–2.5 × 1011 p/kWh), bringing the >10 nm levels of the engines to approximately half of the current particle number limit, applicable to particles >23 nm.