PURPOSE: To select an optimum gear ratio of an AT vehicle by providing input/ output rotary shafts coaxially, and detecting a meshing part of input/output side wheels and thereby detecting a torque of an engine output shaft.

CONSTITUTION: An input side wheel 4 is fixed to the disk-shaped tip of an input rotary shaft 2, and on the input side wheel 4 an output rotary shaft 8 is installed through a bearing 6. The output and input rotary shafts 8, 2 are arranged coaxially with the ends thereof opposing to each other. An output side wheel 12 is fixed for meshing with a spline 10 formed on the output rotary shaft 8. Projection members 24, 26 that is, the meshed parts of the input/output side wheels 4, 8 are on a common virtual cylindrical face outside of which a projection detector 28 is provided in the vicinity thereof. As a result, an output torque is directly measured, which is used as data in selection of a gear ratio of AT vehicle, thereby excellent control can be carried out.