[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.