A method of estimating a combustion index and/or the indicated torque (T ind) in a cylinder of a four stroke internal combustion engine with spontaneous mixture ignition provided with a drive shaft coupled to a phonic wheel presenting a number N of teeth; the method of estimating comprises the steps of: determining the angular speed (ω i) of the drive shaft; determining, by means of a frequency analysis of the angular speed (ω i) of the drive shaft, at least one harmonic (Ω n) of the speed signal characterized by its own module (|Ω n |) and is own phase (Arg(Ω n)); determining an inverse mechanical model (H(jώ)) of the transmission which represents, in the (ώ) frequency domain, the relation between the Fourier transforms of the angular speed (Ω) and of the torque of the internal combustion engine; determining at least one indicated torque harmonic (T ind, n) characterized by its own module (|T ind, n |) and by its own phase (Arg(T ind, n)) by applying the inverse mechanical model (H(jώ)) of the transmission to the speed signal harmonic (Ω n); determining a first algebraic function (Ψ) which puts the combustion index into relation with the phase (Arg(T ind, n)) of the n th indicated torque harmonic (T ind, n) and/or determining a second algebraic function (Γ) which puts the indicated torque (T ind) into relation with the module (|T ind, n |) of the n th indicated torque harmonic (T ind, n); and determining the combustion index by applying the first algebraic function (Ψ) to the n th indicated torque harmonic (T ind, n) and/or determining the indicated torque (T ind) by applying the second algebraic function (Γ) to the n th indicated torque harmonic (T ind, n); the method of estimating is characterized in that the step of determining at least one indicated torque harmonic (Tind,n) uses the following equation: T ind, n = H jΩ n ∗ Ω n − Ω nC + T indC, n T ind, n n th indicated torque harmonic; H(jΩ n) inverse mechanical model of the transmission; Ω n n th speed signal harmonic; Ω nC n th speed signal harmonic in analogous cut-off condition; T indC, n n th indicated torque harmonic in analogous cut-off condition. determining the angular speed (ω i) of the drive shaft; determining, by means of a frequency analysis of the angular speed (ω i) of the drive shaft, at least one harmonic (Ω n) of the speed signal characterized by its own module (|Ω n |) and is own phase (Arg(Ω n)); determining an inverse mechanical model (H(jώ)) of the transmission which represents, in the (ώ) frequency domain, the relation between the Fourier transforms of the angular speed (Ω) and of the torque of the internal combustion engine; determining at least one indicated torque harmonic (T ind, n) characterized by its own module (|T ind, n |) and by its own phase (Arg(T ind, n)) by applying the inverse mechanical model (H(jώ)) of the transmission to the speed signal harmonic (Ω n); determining a first algebraic function (Ψ) which puts the combustion index into relation with the phase (Arg(T ind, n)) of the n th indicated torque harmonic (T ind, n) and/or determining a second algebraic function (Γ) which puts the indicated torque (T ind) into relation with the module (|T ind, n |) of the n th indicated torque harmonic (T ind, n); and determining the combustion index by applying the first algebraic function (Ψ) to the n th indicated torque harmonic (T ind, n) and/or determining the indicated torque (T ind) by applying the second algebraic function (Γ) to the n th indicated torque harmonic (T ind, n); the method of estimating is characterized in that the step of determining at least one indicated torque harmonic (Tind,n) uses the following equation: T ind, n = H jΩ n ∗ Ω n − Ω nC + T indC, n T ind, n n th indicated torque harmonic; H(jΩ n) inverse mechanical model of the transmission; Ω n n th speed signal harmonic; Ω nC n th speed signal harmonic in analogous cut-off condition; T indC, n n th indicated torque harmonic in analogous cut-off condition. T ind, n n th indicated torque harmonic; H(jΩ n) inverse mechanical model of the transmission; Ω n n th speed signal harmonic; Ω nC n th speed signal harmonic in analogous cut-off condition; T indC, n n th indicated torque harmonic in analogous cut-off condition.