System for controlling the stability of a vehicle, the system comprising means for imparting a longitudinal force on the tire and means for calculating the slip parameter G.sup.Opt at each activation of the means for imparting a longitudinal force on the tire in the following manner: determining coefficients A.sub.[avg/p] by direct calculation or by an appropriate regression, from a sufficient number of pairs (.mu..sub.i, G.sub.i), so as to model a first curve of variation .mu..sub.i=f(G.sub.i, A.sub.[avg/p]) including the origin, and the pair or pairs (.mu..sub.i, G.sub.i), in which .mu..sub.i is different from zero, determining an indicator of the average slope .alpha..sub.1 of the first variation curve, determining coefficients B.sub.[avg/p] by direct calculation or by an appropriate regression, from a sufficient number of pairs (.mu..sub.i, G.sub.i), so as to model a second variation curve, free not to pass through the origin, .mu..sub.i=f(G.sub.i, B.sub.[avg/p]) including the pair or pairs (.mu..sub.i, G.sub.i), in which .mu..sub.i is different from zero, determining an indicator of the average slope .alpha..sub.2 of the second variation curve, as long as the difference between .alpha..sub.1 and .alpha..sub.2 is below a predetermined slope threshold, repeating the previous operations for each new acquisition of the pair of values (G.sub.i, .mu..sub.i), as soon as the difference between .alpha..sub.1 and .alpha..sub.2 exceeds the predetermined slope threshold and determining a target slip G.sup.Cavg using at least the last pair of values (G.sub.i, .mu..sub.i).