High emitter vehicles, as a small fractions of vehicles that contribute disproportionately to fleet emissions, play a critical role in urban air quality. This study analysed remote sensing data from two large UK campaigns (2016–2017 and 2022) to quantify fleet-wide real-world emission factors (EFs) for HC, CO, PM, NO, NO2, and NOx, and to identify high-emitting vehicle subsets (HEVS). Using over 184,000 data records, distance- and fuel-based EFs were estimated with bootstrap-derived uncertainty, and cumulative Pareto analysis was applied to characterise skewness in emissions. Over the five-year interval, the fleet composition shifted substantially, with Euro 6 vehicles more than doubling in most categories. By 2022, Euro 6 buses and HGVs showed marked reductions in PM, HC, and CO, largely reflecting the widespread deployment of diesel particulate filters. However, diesel cars and vans continued to produce elevated NOx compared with regulatory expectations. The results reveal highly uneven emission distributions: depending on pollutant and vehicle type, between 12% and 50% of the fleet accounted for 80% of total emissions. Petrol cars displayed acute skewness for HC and CO, where fewer than 20% of vehicles dominated cumulative emissions. Diesel cars and vans showed stronger persistence of high NOx and NO2 emitters, with 30-40% of vehicles responsible for 80% of emissions. Historical comparison with 2016–2017 indicates that high-emitter prevalence has declined, particularly for passenger cars, reflecting both fleet renewal and Clean Air Zone (CAZ) enforcement. These findings indicate that continued real-world monitoring combined with targeted interventions against high emitters is essential for closing the gap between expected and actual emissions and accelerating improvements in urban air quality.