<abstract> <b><sc>Abstract. </sc></b>This article presents investigations of fuel consumption and CO<sub>2</sub> emission of a farm tractor operating with a cultivator. The cultivator has been designed to operate at greater speeds of up to 15 km/h. During the tests, the tractor operated with different speeds, which influenced the fuel consumption and the time of operation. An increase in the cruising speed resulted in greater fuel consumption and a reduced time of operation. A fuel consumption curve was determined depending on the tractor speed. At the same time, the authors assessed the quality of the cultivation through soil analysis after the cultivation. The fuel consumption was measured with the carbon balance method â the most accurate method available. Such measurements were performed by the Portable Emission Measurement System (PEMS) equipment. The investigations utilizing the PEMS equipment enable measurements of the exhaust emissions under actual operating conditions, during fieldwork. The methodology used in the research is the most accurate and most reliable one currently available. It takes into account all of the variables of actual operation. This method is a state-of-the-art achievement in exhaust emissions measurement and has great potential. It may be successfully used for research related to assessment of energy consumption of farm tractors depending on the analyzed factors, e.g. type of soil, design of tools, and method of fieldwork. The performed tests allowed formulating conclusions related to the influence of the speed of the farm tractor with the cultivator on fuel consumption and emission of CO<sub>2</sub>. From the test, it follows that upon reaching a certain operating speed the increment of the fuel consumption is getting smaller while the time efficiency increases. The authors also observed that the above did not have impact on the quality of the cultivation. The presented results of the investigations and their analysis confirm the usefulness of the applied methodology in the assessment of energy consumption of farm tractors and machines during fieldwork operation.