consumed through the first monitor tire from the torque sensor and the rpm gauge and the second monitor tire consumption energy value 2 m t c e that is consumed by the second monitor tire is calculated in a like manner referring to fig1 if the first standard tire consumption energy value 1 r t c e is 30 and the first monitor tire consumption energy value 1 m t c e is 30 the vehicle is in a stable driving state however if the difference value between the first standard tire consumption energy value 1 r t c e and the first monitor tire consumption energy value 1 m t c e is in excess of a predetermined range or a ratio therebetween exceeds a predetermined range the vehicle is in an unstable driving state similarly in various embodiments of the present invention if the first standard tire consumption energy value 1 r t c e is 30 and the second monitor tire consumption energy value 2 m t c e is 30 i e a ratio therebetween is substantially equal to 1 the vehicle is in a stable driving state however if the difference value between the first standard tire consumption energy value 1 r t.