A method of determining a cause of a performance deficiency of a vehicle may include: Determining whether the vehicle is operating in a defined load condition; comparing an actual vehicle performance parameter with a predetermined baseline performance parameter for the defined load condition; comparing a plurality of key indicator values of the vehicle with a predetermined specification for each of the plurality of key indicator values; concluding that the performance deficiency is the result of a mechanical condition of the vehicle when at least one of the key indicator values is outside of the predetermined specification for a corresponding key indicator value and when the actual vehicle performance parameter is outside of the predetermined baseline performance parameter; and concluding that the performance deficiency is the result of an operational condition when none of the key indicator values is outside of the predetermined specification for the corresponding key indicator value and when the actual vehicle performance parameter is outside of the predetermined baseline performance parameter.