Mechanical resistance consists of the rolling tire resistance and power losses of vehicle drive train. Rolling resistance is done by deformation of vehicle tires on road surface. Power losses represent resistances of bearings and wheels rolling and oil wading. This resistance acts continuously during vehicle wheels rotation. The value of this driving resistance is possible to test by different methods. Laboratory tests on rotating drum or dynamometer are usually used. This paper deals about rare method – coastdown exterior driving test. It was done with a casual truck trailer set. Results of this test method show the value of this resistance forces which are converted in to coefficient of mechanical resistance and discussed in the point of view of rolling resistance coefficient values.