A system 10 for determining high accuracy gradient information includes linearization means 20, a sum unit 45, and state space model and observer means 50 for calculating a road gradient .chi.Road. The linearization means 20 includes an air resistance calculation unit 30 and a rolling resistance calculation unit 40. The means 50 includes a state space model 60 and a state space observer 70. The sum unit 45 determines a total sum wheel force .SIGMA.FLij of wheel forces FLij. The linearization means 20 calculates an air resistance force FW and a rolling resistance force FR based on a vehicle velocity vCoG using approximation equations, calculates a linearized total sum force FSum by subtracting the air resistance force FW and the rolling resistance force FR from the wheel forces FLij, and inputs the linealized total sum force FSum to the state space model and observer means 50.