A device is disclosed for pumping the tire of a vehicle while the vehicle is travelling. The device is mounted on the wheel axle and, apart from a hanging counterweight (226) which remains stationary, the entire device rotates with the axle and wheels. Tire pressure is sensed and, when below a predetermined level, a piston (22) moves axially to press a rotating clutch plate (260) against the stationary counterweight (226). The clutch plate (260) thus stops rotating. Part of a pump drive structure (216) stops rotating with the clutch plate (260). Relative rotation between this part of the pump drive structure and the remainder of the pump drive structure causes air to be pumped to the tire. When tire pressure has been restored the piston (22) moves back, the clutch plate (260) is released, and the pumping action ceases.