I have a 2000 SSEI that has been getting terrible gas mileage lately. It is never driven in town, always at least 30-50 highway miles per day. It used to get around 25-26 now it is down to 20 on the high side. It has been as low as 18. I have replaced the plugs and wires and fuel filter. Should I look at the O2 sensor?? I do not always have access to the 92 octane fuel and usually put no less than 89. Would this make a difference??

I would replace the O2 sensor for starters & get back on 92-93 octane fuel, stay away from 89. Do you notice any engine knock?

I agree. If you have over 50,000 miles or so and the O2 has never been replaced, it would be a good idea. Have you done the usual tune up items that will also affect gas mileage? Plugs, wires, air filter, etc.

Yeah, I have done just about everything else. I will change the O2 sensor and see if that helps.

Just in case you haven't search the board - do not get a Bosch or Borg/Warner O2 sensor. They do not last in our cars. AC-Delco, or Denso are the brands that have have been used without issues. The others only last a few months.

Also............where do you live? I know its been very cold up North as I live here,,,,,,my mileage has dropped way off.

Also make sure your car is getting up to temp. An open thermostat can keep the car in fuel enrichment mode and waste a lot of gas.

Would a drilled thermostat have the same effect?

It has been sub zero here for a little over a week and my MPG has taken a nosedive as well.

How large of a hole is drilled into it? It'* possible that it could contribute, but I'd also then expect to see a code thrown that the engine did not warm up quickly enough. It'* not too surprising that the lower temps will make the gas mileage drop. It takes longer for the engine, and the transmission to warm up. The engine gets the richer mixture longer, and the transmission takes longer before it goes into TCC lockup.

what RPM do you have at 75mph? or 60mph?