Fuel Economy Questions

[txsilver]

Dear Mustang Friends,

I have a fuel economy problem in my 2000 Ford Mustang V6. If you could help me answering my questions, I will appreciate.
My problem is, I can only make 210 miles with 13.5 gallons of gas during winter time( which is 15 mile/gallon). Is it 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. And according to 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 with 14 gallons of gas in the same period last year.

[Red Mustang Gt 46]

Some states if not all put an additive in their gas to reduce emissions during the winter months...could reduce mileage.
Tire inflation at 35 lbs. all the way around? (I'm comfortable at 35 lbs. 17" tires)
Air filter clean and sparkly? (K&N cleaned every 6 months)
Fuel filter been changed lately? (every 20,000)
Have you switched to synthetic oil yet? (to gain mileage)
Do you use fuel injector cleaner each time you change your oil? (every 3000)

Just some questions to ask, could be anything.

These are just some of things I do regularly to keep my car happy.