Low and ultra low viscosity oils are one of the main solutions considered in view of the improvement of energy efficiency for better fuel economy.The recent modification of SAE J300 engine oil viscosity classification, to include engine oils with high temperature & high shear rate (HTHS) viscosity of 2.3 mPa·s for the SAE 16 grade, has opened debate on the possible real benefits that could derive, in terms of fuel economy and CO2 emission reduction, from the use of ultra low viscosity oils on engines of current technology.Two European compact cars (C-segment) of recent technology and similar characteristics were employed in our laboratories, on chassis-dyno test bed, to evaluate fuel economy with the use of oils having an HTHS viscosity decreasing from 2.9 to 2.0 mPa·s, with a −0.3 mPa·s step.Different test cycles were selected for this purpose: the New European Driving Cycle (NEDC), applied in laboratory test approvals within EU, that is composed of a urban driving and a high speed driving cycle, and the Urban Driving Cycle developed within the European ARTEMIS project (Assessment and Reliability of Transport Emission Models and Inventory Systems), based on statistical analysis of a large database of European real world driving patterns.The tests were designed according to a statistical approach. By combining the variables “oil”, “driving cycle” and “vehicle” according to a factorial design, 214 chassis-dyno tests were carried out.The results of the experimentation point out different levels of fuel consumption and CO2 emissions for both vehicles, as well as different trends of fuel consumption as a function of HTHS viscosity.Fuel consumption of the first vehicle, showing the best fuel economy, presents an initial downward trend as HTHS decreases, followed by a plateau and a general upward trend. This final negative upward trend is emphasized in the Artemis Urban Driving Cycle.A different behavior occurs on the second vehicle, where a general progressive downward trend emerges when decreasing HTHS viscosity. This positive trend is weakened in the Artemis Urban Driving Cycle.The application of the statistical technique of Analysis of Variance (Anova) reveals that the variable “oil” has a highly statistical significant effect on fuel consumption and CO2 emissions, as well as the interaction between the variables “oil” and “vehicle”. This latter result is the statistical confirmation that the two vehicles provide a different ranking of the testing oils.