A method and apparatus are provided for monitoring tire pressure for motor vehicles, or the like, in which coded sound signals emitted from a wheel-mounted sound signal generator are detected and processed. The coded signal from the generator serves to distinguish the sound signal from background noise. The output of the generator is comprised of at least two coherent frequencies that beat acoustically with one another to produce an amplitude modulated sound signal that is then detected by a transducer which converts the sound signals into the electrical signals. The electrical signals are then fed through the processing circuitry including a detector which extracts the modulation envelope and applies it to an audio band-pass filter. The filter accepts the envelope within a predetermined frequency range, eliminating all signals which are not within the designed band pass. A second detector converts the audio signal to a DC level and feeds it to a threshold comparator. The level of the DC signal is directly proportional to the percentage of modulation of the originally received signal. If this level exceeds the threshold value of the comparator, the output actuates a bistable device, which, in turn, actuates a warning indicator.The rate of loss of tire pressure may also be determined by means of a signal generator adapted to emit at least two time-spaced sound signals at different pressures. The two signals are detected and processing circuitry calculates the time and/or pressure remaining before the vehicle becomes immobile. This information is displayed to the operator.