ive got a 93 del sol,factory stock with d16z6 auto.it was getting around 35 mpg,and in the last couple of years thats dropped to 30.it has new distributor,plugs,wires,air cleaner,fuel filter and 30 PSI air in the tires.it doesnt have any issues such as smoke from the tailpipe and runs fine with 177k miles on it.should i consider changing the o2 sensor or is something else the problem?(original o2 sensor)