In a device for the accurate measurement of the specific fuel consumption of internal combustion engines, which utilizes a scale having a pointer movable in front of a dial, the measuring scale carries detector or pick-up means comprising each a first transistorized oscillator circuit with a frequency modulated inductance-capacitance circuit, a second oscillator circuit generating a fixed reference frequency signal, a frequency changer receiving the first and second oscillator circuits signals, a low pass filter means to which is transmitted a reference frequency signal which corresponds to the difference of frequencies of the signals received by the frequency signal, a signal detector connected to the filter and rectifying through a doubler the HF voltage having passed through the filter and a shaping device connected to the output of the detector for delivering at a low impedance value a high-amplitude logic signal.
