Abstract: A system is disclosed for warning that the pressure in one tire of a pair of dual tires is more than a predetermined amount below the pressure in the other. The system comprises a device fitted to the valve stem of the outer tire and having a single air inlet and two air outlets connected one to each tire. Two pistons are subjected to tire pressures and themselves act on a lever. When the tire pressures differ, the forces on the lever are unbalanced, and the lever tilts. A trigger unit is mounted on the rim of the inner tire and linked to the lever by a stiff wire. When the wire is displaced, a spring loaded arm of the trigger unit is released. On the body of the vehicle there is a switch the operating member of which is struck by the spring loaded arm after that arm has been released. The switch is connected to a warning device in the vehicle cab.