With all the environmental concern of diesel fuelled vehicle, it is a formidable challenge to phase out them completely specifically from Heavy duty application. Most pragmatic solution lies in solutions which improves the fuel economy and reduce the carbon emission of existing diesel fuelled vehicle fleet and have economical & feasible for vast geographical extent of country. With implementation of BS VI emission norms across country from April 2020, supply of BS VI complied diesel fuel started and BS VI complied vehicles with upgraded engine technologies and after treatment devices have started to come which make present vehicle fleets heterogeneous with substantive number of BS IV vehicle. Beside improvement of engine technologies, existing BS IV vehicle fleet performance can be enhanced through improved fuel and lubricants solutions. This research work is a step towards improving the fuel economy of existing BS IV diesel vehicles through the intervention of differentiated diesel fuel and dedicated state of the art lubricants combinations. Developed through modification & upgradation of BS VI diesel fuel, Differentiated Diesel fuel is having excellent injector cleaning properties, improved combustion, low soot formation etc which intern improve the combustion result in fuel economy improvement. and reduce carbon emissions. Further, efforts were made to comprehensively review the lubricant requirement of vehicle and designed & developed dedicated state of the art lubricants combination comprising engine oil, transmission oil and axle oil collectively named as Green Combo lubricants which reduce the friction from engine and drivelines and results in further fuel economy benefit. To evaluate the actual fuel economy improvement and emission reduction credentials, comprehensive field trials of Differentiated Diesel and Green Combo lubricants were undertaken in fleet of heavy duty diesel buses deputed at state transport. State transport was selected due to availability of BS IV complied diesel fuelled buses of varied vintage from 50,000 km to 2,00,000 km and nearby vicinity to have better field trial supervision. Field trials were designed in such a way that combined effect of Differentiated Diesel and Green Combo Lubricants and then effect of Green Combo lubricants alone can be evaluated. Fleet of buses were charged with Green Combo lubricants after proper flushing and put new engine oil filter. After charging the Green Combo lubricants, buses were fuelled with Differentiated Diesel and sent to their routine routes for actual running for evaluating the combined effect of differentiated diesel and Green Combo lubricants for initial field trials and buses covered in the range of 25,000 km to 40,000 km each. Afterwards, field trial buses were fuelled with normal BS VI diesel and completed the field trials for evaluation the effect of Green Combo lubricants only. Used engine oil samples and idle emissions through portable emission measurement system as per field trial protocol were taken during the field trials. 1,50,000 km field trials were successfully completed and it demonstrated that Differentiated Diesel and Green Combo Lubricant together improved the fuel economy by 7% - 9% and emission reduction in CO2, CO, NOx and THC are 4.5%, 10%, 4% and 5 % respectively. Also, Green Combo lubricants improved the fuel economy by 3% - 5%. Used engine oil analysis confirmed the satisfactory performance of Green Combo lubricants during field trials. Based on theoretical assessment of implementation of this combination together at throughout state transport leads to saving of ~ 125 kL Diesel annually and thus ~ 330 Tonne CO2 annually which shows the immense potential of saving the precious diesel fuel and reduced the carbon emission.