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Why’d my mileage drop?

My 2010 F-150 with the 4.8L V-8 was getting a consistent 19.4 mpg for the past 45,000 miles per the onboard computer. Then the check engine light came on & mileage dropped to 18.6.per the computer. I had the two upstream O2 detectors replaced & they “updated the program”. Now my mileage has dropped to 18.2 (per the computer) and to 17.3 (per my old fashioned miles driven/gallons consumed calculation). Now did the mechanics screw something up, has the “new program” cratered my mileage, or was the computer lying to me in the past? Thank You!

It would be helpful to tell us what error code triggered the trouble light.
It has the format Pxxxx, P0123 for example.
What climate are you in? Mileage will be lower in the winter.
It could have another issue, like low tire pressure, wheel mis-alignment, a dragging brake, tired thermostat.
I replace the thermostat and radiator cap every 10 years for preventive maintenance.

All good questions by @circuitsmith … worth looking into. Also buying a different brand of fuel may factor in… We all know and dislike the “winter blend” crap they foist upon us as it contains 1.7% less energy density over the summer stuff. But 1.7% less would only equate to about .3 mpg less.

So yes… there is something afoot here.

Maybe a battery disconnect / full reset of the ecu will produce different numbers… Might be worth a shot…just to see what calculations it comes up with from the point of reset. (prolly not needed, but, might be interesting)

The thermostat may be partially stuck open.


I suspect that truck always got 18.2 MPG in the winter, the display is showing a cumulative average of summer/winter unless it is reset.