When was the last time you physically checked the spark plugs?
If you're using the OEM NGK copper plugs, they only have a 30K lifespan.
I had the same precipitous drop-off in mileage last year.
At it's worst, my city only mileage hit 20.25 mpg.
Turns out the plugs were being fouled by an accumulation of oil.
Changing the seals on the spark plug tubes stopped the fouling problem and with a new set of OEM plugs, it solved the gas mileage woes immediately.
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