ISO 19940:2017 specifies the testing method for determining the tyre stiffness index of passenger car tyres for the products capable of supplying the vehicle with the basic tyre functions, at least, at a speed of 80 km/h (50 mph) and a distance of 80 km when operating in flat tyre running mode, as per ISO 16992. This method is meant to determine the above mentioned index, for the characterization of the expected tyre's stiffness through its air and structural components. To reach the target, the vertical force and the vertical deflection, in the sense of absolute position in Z direction under different operating conditions, are measured (approximated in case of zero inflation pressure) and combined through the metrics defined in this document. This method is not intended to be used for conventional tyres.