A device (10) for providing air under pressure to a rotating pneumatic tire is disclosed. The device comprises a pump (48, 50, 58) which in use rotates with the tire and which provides air under pressure to the tire when the pump is activated. There is a pump drive system for activating the pump upon a loss of pressure in the tire. The pump drive system includes a connecting rod (52) which is rotationally connected to the tire and an eccentric body (32) which rotates with the rod when the pump is inactive. The eccentric body is inhibited from rotating with the rod to activate the pump. Relative rotation between the rod and the eccentric body drives the pump to provide air under pressure to the tire. An electromagnetic clutch is provided which has a first plate (26) which is non-rotatable and a second plate (36) which rotates with the body. The first plate is connected to the second plate electromagnetically when the clutch is engaged to prevent rotation of the second plate and hence of the connecting rod.