Air-fuel ratio control method for internal combustion engines

In an air-fuel ratio control method for an internal combustion engine installed on a vehicle, said engine having an intake passage, an exhaust gas ingredient concentration sensor arranged in said exhaust passage for producing output substantially proportional to the concentration of an ingredient in exhaust gases emitted from said engine, and a transmission, wherein the air-fuel ratio of an air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio which is set to a value leaner than a stoichiometric air-fuel ratio, by the use of said output from said exhaust gas ingredient concentration sensor, depending upon operating conditions of said engine, and said desired air-fuel ratio is set to a leaner value as said transmission is set to a smaller reduction ratio,

• the improvement comprising the steps of:
• (1) detecting a reduction ratio to which said transmission has been set;
• (2) detecting travelling speed of said vehicle;
• and
• (3) setting said desired air-fuel ratio to a richer value as said travelling speed of said vehicle is lower, insofar as said transmission is set to the same reduction ratio.