It is expected that the world's energy demand will double by 2050, which requires energy-efficient technologies to be readily available. With the increasing number of vehicles on our roads the demand for energy is increasing rapidly, and with this there is an associated increase in CO₂ emissions. Through the careful use of optimized lubricants it is possible to significantly reduce vehicle fuel consumption and hence CO₂.This paper evaluates the effects on fuel economy of high quality, low viscosity heavy-duty diesel engine type lubricants against mainstream type products for all elements of the vehicle driveline. Testing was performed on Shell's driveline test facility for the evaluation of fuel consumption effects due to engine, gearbox and axle oils and the variation with engine operating conditions.To complement the rig-based testing, a field test protocol has been developed to better understand the linkage between operating conditions and fuel economy changes when driveline lubricants are changed. Two standard, delivery-type, 18-ton trucks have been modified with fuel-flow, engine operation and GPS (global positioning system) measurement equipment. Following a fixed on-road test route in the UK containing city, local, highway and hill driving conditions, the fuel consumption data was separated out for each situation to demonstrate the relative differences. To minimize variability, tight controls were placed on the test set-up, measurements taken and the operation of the vehicles.Using a Mercedes Benz OM 460LA heavy-duty diesel engine run under the World Harmonized Transient Cycle (WHTC) and World Harmonized Stationary Cycle (WHSC), a combination of a SAE 5W-30 engine oil, SAE 75W-80 gearbox oil and SAE 75W-90 axle oil gave average fuel consumption improvements of 1.8% and 1.1%, respectively, relative to a SAE 15W-40 engine oil, SAE 80W gearbox and a SAE 90 axle oil. Using the WHSC cycle, significant variations in the individual lubricant contribution under different speed/load conditions within the cycle were identified. Additionally, an average fuel consumption improvement of 1.8% was observed using medium-duty trucks under a range of typical European driving conditions in a controlled field trial.