 For this reason, European WLTP results from PEMS will be weighted with a conformity factor of 2.1 (1.5 after 2019), i.e.the emissions measured by the PEMS are allowed to be a factor 2.1 higher than the limit.It is expected that a variety of on-board systems will be designed, ranging from bread-box sized PEMS to instrumented trailers towed behind the tested truck.