NOVELTY - The pressure indicator comprises pressure sensors, processing chips, transmitters, batteries and wires (52) in a component comprising gaskets and washers for a cap that closes an opening of a fuel tank, a movable washer arranged in the fuel cap, and a unit for providing a humanly perceivable notice when the signal is received by a receiver (46), that is disposed on a key ring (44). The processing chips cause the transmitters to send a signal to the receiver when the sensors detect that the pressure in a fuel tank is below a predetermined level.    USE - The pressure indicator is useful for motor vehicle fuel tank to indicate when pressure at the cap/container interface or vapor pressure in the tank drops below a predetermined level.    ADVANTAGE - The pressure indicator is capable of efficiently indicating when the pressure at the cap/container interface or vapor pressure in the tank drops below the predetermined level thus enhancing the efficiency of the motor vehicle and reducing the generation of heat during the operation of the motor vehicle.    DETAILED DESCRIPTION - The pressure indicator comprises pressure sensors, processing chips, transmitters, batteries and wires (52) in a component comprising gaskets and washers for a cap that closes an opening of a fuel tank, a movable washer arranged in the fuel cap, and a unit for providing a humanly perceivable notice when the signal is received by a receiver (46), that is disposed on a key ring (44). The processing chips cause the transmitters to send a signal to the receiver when the sensors detect that the pressure in a fuel tank is below a predetermined level. The batteries supply power to the sensors, the processing chips and the transmitters. The wires electrically connect the batteries, the sensors, the processing chips and the transmitters. The pressure in the fuel tank is below the predetermined level indicated on a fuel cap by a wheel that turns with change of pressure when the signal is received by the receiver. The wheel has a red area indicating low pressure, and a green area indicating normal pressure. The red or green area is displayed through a window in the fuel cap. The fuel cap emits a sound indicating when the cap is screwed to a correct tightness and when the cap is replaced to cover the fuel tank after refueling. An alarm is sounded when the cap is not screwed to the correct tightness. An inside of a door covering the fuel cap has a marking that indicates the correct position to which the fuel cap is rotated to close it and seal the fuel tank. The movable washer seals the fuel cap when it is loose, and is activated when a user pushes a button after an indicator shows that the fuel cap is loose after the receiver receives the signal. The pressure sensors in the component measure: pressure between a lip of an opening of the fuel tank and the cap that closes the opening; and vapor pressure within the fuel tank. The fuel tank is arranged in a motor vehicle. The gasket is made of rubber. A Bluetooth technology is used to send the signal. The humanly perceivable notice is a flashing light (48), and consists of sounds and vibrations.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a first receiver/indicator.    Rectangular housing (42)    Ring (44)    Receiver (46)    Light (48)    Battery (50)    Wires. (52)