I originally got 371 miles after premium fill-up. I now get 350 miles after premium fill-up. What would make the mileage lower? What could be the problem with the car?

 

Well, its only 21 miles per tank. Could be the type of driving mix (city versus suburbia versus interstate). Idling more at lights / turns / shopping etc. For 21 miles per tank, would be a non worry for me personally. And I think there is a different formulation for gasoline in winter months too. Even for North carolina.

 

We get winter blend gas around here which negatively effects gas mileage.

 

BINGO! 10% ethanol = 10% decrease in mileage, etc.. Happens every winter.

 

We have 10% year round and I use regular in my 2010. Always get 400 miles to a tank unless I have more highway driving.

 

10% all year around here too. They still do a winter blend in addition to the ethanol.

 

20 miles per tank = 20 miles per 18 gallons = ~1.1mpg difference.

Checked your tire pressures lately?

 

[/SIZE]"]What pressure do I have checked for lower mileage after fill-up?[/SIZE]

 

Tire pressure. There is a sticker on the inside of the drivers door jamb with the recommended tire pressure for your car. Under inflated tires can have an adverse effect on gas mileage.