A method and a system for detecting a fuel quality in a vehicle including a first determination unit to determine a correction factor kkW for fuel as a quotient between an output fokW required to propel a vehicle and a reference output engkW which an engine in the vehicle is estimated to yield; k k ⁢ ⁢ W = fo k ⁢ ⁢ W eng k ⁢ ⁢ W ; a second determination unit to determine a correction factor kNOx for exhaust gas emissions as a quotient between a value measured in the vehicle for nitrogen oxides engNOx and a reference value for nitrogen oxides ECUNOx; k NOx = eng NOx ECU NOx ; and a detection unit to detect the fuel quality based on a relation between the correction factor kkW for fuel and the correction factor kNOx for exhaust gas emissions.