The CO2 reduction required by legislation represents a major challenge to the OEMs now and in the future. The use of fuel consumption saving potentials of friction-causing engine components can make a significant contribution. Boundary potential aspects of a combustion engine offer a good opportunity for estimating fuel consumption potentials. As a result, the focus of development is placed on components with great saving potentials. Friction investigations using the motored method are still state of the art. The disadvantages using this kind of friction measurement method are incorrect engine operating conditions like cylinder pressure, piston and liner temperatures, piston secondary movement and warm deformations which can lead to incorrect measurement results compared to a fired engine. In the past, two friction measurement methods came up, the so called floating liner method and a motored friction measurement with external charging. Both methods are getting closer to realistic engine load and thermal distortion, nevertheless there are still shortcomings. The floating liner method can only be used with great effort on a single cylinder research engine with cylinder pressure limitation. Using the motored method with external charging, cylinder pressure maxima can be reached, but with incorrect piston and liner temperature and an incorrect phasing of the cylinder pressure curve. In this paper a newly developed friction measurement method which allows a detailed strip down of a fired diesel engine for a complete engine map is used for the measurements. With this method the total friction can be divided into shares of the coolant pump, belt drive, oil pump, balance shafts, cranktrain, high-pressure pump and valve train. The measurement method was tested and verified with a state-of-the-art 3-cylinder diesel engine for passenger cars. The experimental results are presented in this paper. Furthermore the influence of different cylinder liner honings on the cranktrain friction was analyzed and compared in legislation cycle relevant engine operating points.The second part of the paper deals with friction simulation. The simulation model used is an EHD (elastohydrodynamic) model to simulate the friction in the contact area between piston skirt and cylinder liner using the AVL software Excite Power Unit. The simulation model was parameterized with detailed input data such as surface roughness, piston and cylinder liner geometry, cold and warm deformations of piston skirt and cylinder liner, cylinder pressure and oil data. Using the EHD simulation model, basic relations of various parameters affecting the piston skirt friction can be investigated. Finally a friction optimized combination of piston skirt and cylinder liner geometry was investigated and analyzed in detail. The combination of EHD simulation and experimental results using the new friction measuring method allows a detailed estimation of CO2 saving potentials resulting from friction reduction measures.