why is the mileage declining on my 2008 prius
Are you operating on the gas engine more? Such as longer trips on the highway?
Tester
All I can say is that my gas mileage on mine (same year) is just as good as usual.
It is getting colder out.
Yup. The colder weather means that your tire pressure has dropped by perhaps as much as 4 or 5 psi. That alone could account for a drop in gas mileage.
Tell us a little more:
- How long have you had your Prius?
- When did you notice the declining mileage?
- How many miles on your prius?
- How are you measuring the decline?
- How much of a decline?
With a Prius, the real issue with colder temperatures is that the gas engine has to run longer to reach full temperature.
It could be one of many possible reasons:
-You changed your route.
-You are computing your fuel economy incorrectly or imprecisely.
-Someone is stealing your fuel.
-One of the drivers of this vehicle is driving aggressively.
-The weather in your area is getting colder.
-The tire pressure is too low.
-The weight of your cargo has increased.
Just how much has your fuel economy declined? How are you measuring the fuel economy?
The last of the unicorns died recently, so they are no longer able to use unicorn remains to make the proper fuel for your little Prius motor. 
Actually, the real reasons why your gas mileage might be dropping is quite numerous.
Did you have your tires replaced recently?
If you did, did you buy the exact tire that came with the car from the factory, or something else?
The tires from the factory are designed to get good fuel mileage, and nothing else. Any other tire will reduce your fuel mileage.
Also, the next glaring reason why gas mileage goes down is that the winter blend gasolines are now reaching the fuel pumps. That gas alone gets lower gas mileage than summer blends, and you might be getting reduced mileage that way also.
BC.