My car is stock as far as engine goes. Fluids are swapped every 5k (5w20 motorcraft blend) tires are inflated to 40psi 99.9% highway -3hr commute 6min back roads. Computer Avg matches the calculated avg 30mpg @70mph. Car has 117xxx miles on the clock. Replaced the oxygen sensor with a motorcraft part from rock auto $70 shipped. 1/3 what my parts dept wanted. Gained 1mpg back with it. I cant seem to increase the mileage even when "drafting". $3.99 in michigan is killing me right now. 60xxxmiles ago i was pulling down 42mpg.
all I can think of is your motor is not working as efficiently due to less pressure from being worn out a bit. Maybe try an engine treatment with your oil. I know this helped my last vehicle with 168k on it
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