A clogged air cleaner will cause the fuel mixture to become richer, resulting in greater fuel consumption and reduced engine performance. The oxygen sensor is part of the system that regulates the air fuel mixture. Oxygen sensors do not die with a bang and they will linger on in a functional stupor for a very long time. If you have over 50,000 miles on your car and you have not replaced the oxygen sensor, give it a try, you might be surprised at how your car runs better and gets better mileage. Contrary to past claims, simply driving slow will not guarantee maximum fuel economy.
Automobile engines have a range where they are most efficient.
Driving in this range will yield the best fuel economy for the driving environment you're in. Under laboratory conditions, your car might achieve its best mileage at 43 miles per hour. However, this is not a practical speed for driving on the Interstate. What you may discover is that your car gets its best mileage on the Interstate if you hold a steady speed of 65 or 70 mph. Your fuel consumption may increase by driving slower or faster than this speed.