The increasing number of vehicles on the road, particularly in the big cities, has considerable negative effects on the environment in terms of toxic emissions. Although, there have been significant improvement with the development of catalytic converters, electronic injection equipment and other advanced systems for on-road vehicles; however, the burden on environment caused by road transport is still high and constantly increasing. Under these circumstances, substitution of conventional fuels by the "clean fuel" natural gas could help reduce emissions and lead to an effective improvement of air quality, particularly in urban areas. Natural gas can be used in normal four-stroke spark ignition engines without any technical problems. Diesel engines, however, require to be converted to CNG combustion. Indian Oil Corporation limited has been always in the forefront for utilization of cost effective alternative energy options endowed with benefits to environment. Under this program we have done a retro fitment in diesel vehicle with CNG Kit which is capable of regulating the amount of CNG with intake air in engine during combustion. In the present study we have generated baseline fuel economy (FE) and Emissions data for vehicle without retro fitment and then completed the required fine tuning, adjustment and mapping of Diesel-CNG dual fuel kit installed in vehicle. The validation of mapping data was afterwards done on the chassis dynamometer facility which provided an idea on regulation of CNG in intake air for desired performance of vehicle. Road trials have been taken after the modification suiting the dual fuel operation and final performance evaluation was done for fuel economy & Emissions on chassis dynamometer. During the comparative studies on performance evaluation in same vehicle with & without modifications it was found out that combustion of natural gas combined diesel fuel in an engine leads to overall fewer toxic substances. As compare to normal diesel operation study found that increase in Carbon monoxide (CO) under dual fuel operation. Dual fuel mode performs better on decreasing in NOX, carbon dioxide (CO2) and PM emissions. In terms of power dual fuel mode produces less power as compare to normal diesel mode.