poor MPG 93 mazda b2600i

i have a 93 mazda b2600i 2WD 5-speed and im getting 10 MPG around town now i do drive everyday and long distances no highways i fill up reset trip switch to 0000 then i drive it and i go to job and come home im at 050.0 lets say and im at a half of tank idk what is wrong its fuel injected i replaced spark plugs and wires changed fuel filter and reset the timing now idk if it is the o2 sensor or the fuel injecter is off

I'd replace the O2 sensor and maybe check the air filter.

I replaced the air filter with a spectra

Upstream o2 sensor right?