After touchdown, the pilot applied gentle braking but believed the aircraft was not decelerating. To avoid running off the end of the strip he attempted a ground loop, however, the aircraft ran off the side of the strip and struck a derelict vehicle. An inspection of the aircraft did not reveal any pre-existing defect that could have contributed to the occurrence. It is probable that the pilot misjudged the speed of the aircraft and the distance to the end of the strip when applying the brakes during the landing roll. He was not wearing the sash component of the seat belt and received facial injuries when he struck the control column during the collision with the vehicle.