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The resistance of indium-oxide covered polyethylene terephthalate foils (IO-PET) shows an extreme sensitivity to tensile strain. In terms of the deformation-resistance transduction, the gauge factor as high as about 60 000 was recorded upon the relative elongation up to 1%. Except the onset of deformation, the nearly exponential dependence of the resistance on strain suggests that the conductivity of the strained films is governed by tunnelling mechanism; this notion is supported by the formation of scattered cracks in the IO- PET film. The cracks are oriented perpendicularly to the strain vector and are characterized by a rather similar and uniform width. Appropriateness of the standard definition of the gauge factor for strain sensors, which are governed by tunnelling conductance, is critically discussed.