I got a 07 2.3 manual transmission ranger. It’s got 168k miles on it. I’ve been tracking my fuel economy and it’s been averaging around 20-21. That seems a bit low to me. Any common things I should check? I’ve already done plugs, wires, and an air filter

 

Change the O2 sensors, only sensors that wear out, 12 years or 100k miles and they need to be changed, no code just a slow drop in MPG

Check odometer, if that's what you are using to calculate MPG, that can be as much as 5% off from the factory, if you are not running Stock tires size then 10-15% off
(GPS is only accurate on straight flat roads)

Not sure if they run Winter gas where you live, but that can knock 1-2 MPG off