Electrically conductive polymer composites (ECPC) are shown as prospective large-size flexible pressure and stretch sensors for detecting of dangerous deformations and vibrations of vehicle parts. Reversible change of resistance dependent on stretch and pressure is obtained in electro-conductive polymer nanocomposites. At certain concentrations of carbon nano-particles a change of electrical resistance by more than four orders is observed at 40% relative stretch. The maximum sensitivity of nanocomposites is observed in the vicinity of the transition of electro-conductive percolation. Nanocomposites exhibit a very weak semiconductor-like temperature dependence of resistance. The tenso-resistive and piezo-resistive effects are found to be practically thermally stable in the region of 20–70 ◦C. A model description of the microstructure providing extremely strong and reversible tenso-resistive and piezo-resistive effects is proposed on the basis of atomic force microscopy of the conductive surface network of the composite. Reversibility of the effects is explained by higher mobility and stronger adhesion of carbon nano-particles to the polymer matrix compared to cohesion between them. The experimental data for tensile strain are in good agreement with theoretical equations derived from a model based on the change of particle separation under applied stress.