Why has the MPG fallen from 20 to 17 mpg?

Possibly its in need of a tune up start with a new air filter and the book shows plug replacement at 90,000 miles

The problem could be the way you are handling the car as well. 20mpg sounds low to me, and 17mpg is abysmal. In the winter, I get about 26mpg, and in the summer I get 31mpg with my 2006 Milan Premier. I live in MN and our roads aren't that great, so I assume people should get at least as good of mileage as I do.

Have you found out why your mercury mpg was decreasing? I recently purchased a Milan 07 V6 premier and it has been doing the same thing to me. If you can hop it would be very appreciated THANKS