A vehicle road way coastdown virtual test system with compensation of wind speed & direction and road slope was developed to satisfy the requirements of reducing environmental influences and improving test accuracy and efficiency. Firstly, based on the American SAE J-2263 standard and the sixth stage’s fuel consumption test standard of China, the CompactRIO was employed in the test system as the key controller which was made by American National Instrument Company. The test method of wind speed and direction, road slope, tire pressure and temperature, etc., and the sensor layouts were systematically introduced. Secondly, the systematic data processing method was proposed, including the connection of segmented coastdown test data, the compensation of wind speed, the elimination of abnormal data, and the coastdown model building based on the test system. Finally, the test system was installed on a passenger car and the systematic coastdown tests were conducted. All the tests indicated that the test system worked reliably and met the test precision requirements. The processing results of the test data showed that the repetition of the coastdown resistances with the compensation of wind speed & direction and road slope was 2.2%. And the systematic deviation between the compensated and none compensated coastdown resistances was 3.7%. The designed coastdown system improved the test precision of coastdown resistance effectively and provided the means for vehicle resistance compensation for the vehicle performances test on the chassis dynamometer.