Method for estimating the ratio between the upstream pressure and the downstream pressure in the turbine of a motor vehicle turbocharger, characterized in that it comprises the following steps: obtaining an estimate of the mass flow of the turbine, of the rotation speed of the turbine and of the upstream temperature of the turbine, as well as the measurement of the position of the variable-geometry blades of the turbine and of the downstream pressure of the turbine, initializing the estimator with the estimated value of the pressure ratio at the present instant of calculation, estimated at the preceding instant of calculation or, if it is not available, with a measurement of the upstream pressure of the turbine or with a predetermined value obtained from a map, determining the value of a first internal variable on the basis of the estimate of the mass flow of the turbine, of the rotation speed of the turbine and of the upstream temperature of the turbine, as well as the measurement of the position of the variable-geometry blades of the turbine and of the downstream pressure of the turbine, together with the initialization value of the estimator and a calculation function, determining the value of a second internal variable on the basis of the estimate of the mass flow of the turbine, the rotation speed of the turbine and the upstream temperature of the turbine, and on the basis of the measurement of the position of the variable-geometry blades of the turbine, the downstream pressure of the turbine, the sum of the initialization value of the estimator and a pressure ratio variation and the calculation function, determining the derivative of the calculation function with respect to the pressure ratio variation, on the basis of the value of the first internal variable, of the value of the second internal variable and of the pressure ratio variation, and obtaining the estimated value of the pressure ratio at the next instant of calculation on the basis of the derivative of the calculation function with respect to the pressure ratio variation, the value of the first internal variable, and the initialization value of the estimator. obtaining an estimate of the mass flow of the turbine, of the rotation speed of the turbine and of the upstream temperature of the turbine, as well as the measurement of the position of the variable-geometry blades of the turbine and of the downstream pressure of the turbine, initializing the estimator with the estimated value of the pressure ratio at the present instant of calculation, estimated at the preceding instant of calculation or, if it is not available, with a measurement of the upstream pressure of the turbine or with a predetermined value obtained from a map, determining the value of a first internal variable on the basis of the estimate of the mass flow of the turbine, of the rotation speed of the turbine and of the upstream temperature of the turbine, as well as the measurement of the position of the variable-geometry blades of the turbine and of the downstream pressure of the turbine, together with the initialization value of the estimator and a calculation function, determining the value of a second internal variable on the basis of the estimate of the mass flow of the turbine, the rotation speed of the turbine and the upstream temperature of the turbine, and on the basis of the measurement of the position of the variable-geometry blades of the turbine, the downstream pressure of the turbine, the sum of the initialization value of the estimator and a pressure ratio variation and the calculation function, determining the derivative of the calculation function with respect to the pressure ratio variation, on the basis of the value of the first internal variable, of the value of the second internal variable and of the pressure ratio variation, and obtaining the estimated value of the pressure ratio at the next instant of calculation on the basis of the derivative of the calculation function with respect to the pressure ratio variation, the value of the first internal variable, and the initialization value of the estimator.