A system and method for self-adjusting engine performance parameters in response to fuel quality variations that includes an exhaust sensor for measuring a level of carbon dioxide present in an exhaust manifold, at least one of a knock sensor and a cylinder pressure transducer for determining a location of peak pressure and a centroid, respectively, a controller in communication with the exhaust sensor and the at least one of the knock sensor and the cylinder pressure transducer, the controller correlating a methane number of the fuel used by the engine to a brake specific carbon dioxide value calculated using the level of carbon dioxide measured by the exhaust sensor and the at least one of the centroid and the location of peak pressure, and an adjusting mechanism, wherein the adjusting mechanism adjusts an engine performance parameter based on the determined methane number.