Vehicles are one of the main sources of pollution in India, which produce substantial amount of pollutants. Gaseous pollutants are reason for major health problems; hence emission legislations are becoming increasingly stringent all over the world. India is also following the global trend of migrating in the Off-highway segment from Trem IIIA to Stage V legislation by 2024. This legislation change is calling for technological upgrade of all existing engines. EGR has been successfully proved as a useful technology to reduce NOx by decreasing the oxygen concentration and the peak temperature of the combustion. Due to compact design and space restriction, the distance required for the homogeneous mixing of fresh air and EGR is not enough. Therefore, the mixing of the EGR and distribution of the EGR over the cylinders may not be equal. This non-uniform mixing of the EGR and the fresh air leads to increase in the NOx and soot compared to the homogeneous mixture due to lower combustion efficiency.1D simulation approach is used for system level studies due to benefits of less simulation time and more information at system level but this approach lacks model accuracy with respect to flow dynamics at component level. 3D flow dynamics of EGR mixing and distribution cannot be analysed with 1D modelling alone. On the other hand, 3D CFD simulations are very useful for component level studies due to detailed modelling, but this approach requires much higher simulation time, making it more difficult to evaluate many design options and boundary conditions. To find a trade-off between the advantages and disadvantages of the 1D and 3D simulation approach, a 1D - 3D coupled simulation approach can be used. In 1D - 3D coupled simulation, complex flow components are modelled in 3D and the rest of the components are modelled in 1D.In this paper, intake manifold and EGR mixer is optimized for uniform distribution of EGR over the cylinders meeting the targets of intake manifold pressured drop, simple design for manufacturing and engine performance. 1D software GT power and 3D CFD software Converge-Lite is used for coupling. This study is carried out for 4-cylinder turbocharged Trem IIIA engine which is being upgraded for Stage V emission norms for off-highway applications. 1D-3D CFD coupling is used to optimize EGR distribution over the cylinders.