More than 600 passenger cars were measured during winter and spring at the end of 2012 beginning 2013 in Antwerp Belgium in order to determine the effect of mileage and year of first registration on smoke emissions or smoke opacity from diesel light duty vehicles. All measurements were contacted in two different campuses of the University of Antwerp and under different ambient conditions. Various parameters such as ambient temperature, ambient humidity, car age, number of passengers in each cars, time of measurement etc. were taken into account and presented in this paper. All findings from these set of measurements were split into two big categories, those cars with less than 150,000km on the clock and the second one with those cars with higher number of mileage.The effect of ambient temperature but also the mileage on smoke opacity will be presented and discussed in more details. Smoke emissions are dependent also on ambient humidity and in this study will be considered the important role that humidity plays in total smoke opacity from various diesel engines. Finally the effect of mileage on smoke opacity is something that will be also examined and discussed intensively. Last but not least, a comparison between two similar cars equipped with the same engine in terms of capacity but fitted with standard and tuned electronic control unit (ECU) respectively was done and interesting findings will be shown in this work.