Poor Fuel Economy. Any solutions?

[txsilver]

Dear Mustang Friends:

I have a fuel economy problem in my 2000 Ford Mustang V6. If you could help me by answering my questions, I will appreciate it.

My problem is that I can only make 210 miles with 13.5 gallons of gas during winter time (which is 15 mpg). Is that normal? Or is it below average?

I took the car to the dealer and it was checked through all the diagnostics and they told me that everything seems OK. They could not find anything wrong with it. According to the diagnostic check my car is consuming 21 miles/gallon. But as I told you above, I can hardly make 15 miles/gallon.

What might be the problem?

P.S. I used to make 240 miles with 14 gallons of gas in the same period last year.