well..usually stock AFR at idling is 14.5... , if u want to save fuel, go for lean burn technic and try reduce engine load... etc.. set to 15 or 15.5 at idling, if ur engine not shaking... then try use alu pulley, electric water pump, elec oil pump, electric fan instead of fan clutch type (certain model car only), water cool air cond pump (throw away the ori air compressor), lightweight flywheel n clutch, engine fully balance, piston+ piston wall+bearing do a 0 friction coating, exh banana do a ceramic coat. no spare tyres, cabon fibre bornet and back trunk and door panel.
with those kind of mod l am 100% sure ur car can run faster than b4 and FC will reduce 20-30%.. for guidence only..