Methods of estimating tractor performance including power efficiency and fuel consumption under field operational conditions were discussed based on a tractor and a large square baler. Methods recommended by ASAE standards were used and results indicated that ASAE standards predicted 15% higher fuel consumption compared to that tested by Nebraska tractor test center. Field trials were conducted in switchgrass fields to verify and demonstrate these methods. Fuel consumption and the material capacity of the tractor-baler system were measured and also estimated using ASAE standards and Nebraska tractor test results. Field test results indicated that the fuel consumption of baling switchgrass was approximately 60% of the fuel consumption estimated with ASAE standards and tested by Nebraska tractor test center. Measured material capacity of the large square baler was approximately 40% of its theoretical capacity. Therefore, the power performance and field capacity of the tractor and baler system might be under used.