Just replaced my upstream O2 sensors, due to poor fuel mileage from rebuilt engine. No check engine codes, just wanted to see if mileage would improve. On the old O2 sensors, I averaged 16.7 MPG per tank of fuel. I’m now averaging 19.6 MPG per tank with the new sensors. I drive the same route every day, so conditions should be the same. Old sensors had 259,000 miles on them, so well beyond their service life. I’d say if your high miles rig is suffering poor fuel mileage, then replace those upstream O2 sensors.