AbstractMost developing countries still have various makes of petrol-driven cars dominating the overall passenger vehicle fleet. In such countries, the emission certification policy for in-use vehicles remains an area of concern, making the inspection and maintenance (I/M) program less effective. A thorough investigation of the exhaust emissions from such cars is required to explore and address this concern. This paper provides insights into the effects of vehicle variables on tailpipe emission parameters from an exclusively larger and heterogeneous dataset of in-use cars (n=1,580). Results showed that both vehicle variables such as age, mileage, emissions norm, and maintenance category, and two engine variables, i.e., aspiration type and fuel mixing conditions, had a significant and direct influence on tailpipe parameters, namely, CO, HC, CO2, O2, λ, and AFR (carbon monoxide, hydrocarbon, carbon dioxide, oxygen, lambda and air-fuel ratio respectively). Stronger correlations were found for the relatively larger (considering age, R2 for COidle=0.88, HCidle=0.73, λf.idle=0.74, AFRf.idle=0.73 and considering mileage, R2 for COidle=0.75, HCidle=0.67, λf.idle=0.62, AFRf.idle=0.61 for the whole dataset) and diverse make-wise (R2 values fared even better, 0.87–0.93 for CO and 0.69–0.77 for HC) data collected during the study. The present research provides a first-hand and comprehensive analysis of the effects of the stringency of emission norms and maintenance category on the exhaust emissions from in-use cars. The polynomial emission equations generated by this study can reliably predict the emission levels for CO and HC based on the age and/or mileage of cars. Further, the results recommend revised policies to upgrade the existing emission certification infrastructure and phasing out policy of cars.