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I've noticed a 2-4 mpg drop in the 2006 Prius in the last couple of months-any ideas? No codes, no "check engine" and no reason to think it is a change in the ethanol content in the Shell 87 octane I always use.Our other vehicle-a real fuel sucker- has actually been getting slightly better mpg lately(for unknown reasons, but "gift horse")

What could it be?
Tire pressure is fine, and it doesn't pull left or right when I release the wheel.No alignment done-no changes in any respect.

It does seem to take longer to go into all electric mode.
Could the O2 sensor be slowly degrading(doesn't it have a built in heater?)?
Common causes of a 5-8% drop in mpg?
2006 ,30,500 miles all city.
Thanks,
Charlie

Check your air filter:leaves, mouse nest, etc.
Have you had your oil changed? (overfull)

msirach,
Thanks for the tips. The oil is ok-maybe .3" down from the top of the stick.The air filter does look fairly sooty/dark with a tiny bird feather in it.
I'll change the filter and see if it helps. We do so little full throttle driving,that I don't know if that will help, but with drive by wire, and the hybrid nature, maybe that throttle plate is more open than my "foot" thinks it is.

Thanks,
Charlie
PS It is a tiny filter!