o2 sensor gas mileage

It was upstream, and I went with Denso. Like stated fix any exhaust leaks before cats, change O2 if it is bad and that should help a lot.

 

Bad upstream O2 sensors will absolutely affect MPG because they are not controlling the Air Fuel Ratio as they are supposed to. This is known as being in "Open Loop" fuel mgmt strategy by the engine computer - which is essentially a default table fuel map for generic conditions (e.g, 70F ambient temperature and sea level atmospheric pressure and 199F engine temp).

When the upstream O2 sensors are working correctly, they send a signal to the engine computer which uses the information to precisely adjust fuel trims in what is known as Closed Loop. Being in Closed Loop optimizes fuel economy, corrects AF ratio for those instances when the engine needs "rich" conditions such as when towing something uphill, and more

If you have bad O2 sensors, they need to be replaced with genuine Toyota sensors. Do not use knock-offs from Amazon or FleaBay.

Downstream O2 sensors monitor catalytic converter efficiency and are less critical. But for the cost compared to the converters themselves, they should also be replaced if they go bad.

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Exhaust leaks will also affect MPG if they are upstream of the upstream O2 sensors in a very bad way. Exhaust leaks result in excess air (oxygen) in the exhaust stream and the O2 sensors will detect this and generally ADD fuel because of a detected "lean" condition when you don't really have one. This will definitely lower your MPG and can foul and diminish the life of your cat converters. I.e., eventually your downstream O2 sensors will throw a code for low cat system efficiency and that is one expensive repair.

HTH