What are the most common causes of decreasing mpg in a 2001 Prius?
Change in driving style.
Incorrect tire pressure and/or wheel alignment. Change in tire brand or type. Ethanol in gasoline. Need for service. Change is use pattern. Increase in weight carried. Change in weather.
How are you calculating fuel mileage, and how much has it decreased?
Check older posts, we get this question a lot. Here is one of the more recent ones: http://community.cartalk.com/posts/list/1005809.page