O2 sensor good gas mileage still bad

[gearheadE30]

Wouldn't hurt to run injector cleaner in the fuel for a few tanks either, although I don't think that is your only problem. How do the plugs look?

[Brew]

Does it look like its been done recently? If not, definitely replace it to rule it out. A bad coolant temp sensor could also be causing a rich fuel mixture.

[pepper]

going through the paces trying to track down the source of an 18-20 mpg problem in my 168,000 mile 318is. Used to get 30-33 mpg but now its way down. Reading the O2 sensor could be a source I went and tested it via the Bentley method checking the resistance and it was right in spec. In the print it says that old sensors react more slowly to changes in O2 than a new sensor and even when the old sensor has the right resistance. The thermostat is an 88C, Air filter pretty new. Even though the O2 sensor tested ok should I replace it?