The increased oxygen content in blends of gasoline with bioethanol causes the necessity for increasing fuel supply to the engine. Car oxygen sensor, reacting to the presence of oxygen in the exhaust gases, increases the amount of injected fuel. Consequently, the higher concentration of bioethanol in fuel blends usually also increases fuel consumption. This study explores how an increase in bioethanol concentration in fuel blends affects the standard car’s fuel consumption and determines which elements of the system limit the maximum possible concentration of bioethanol in the blend with gasoline.