It should improve the more you drive and the ECM re-learns with fresh data. Battery dis-connect will restore default settings until it learns your habits.

 

Thank you, that's what I assumed but didn't know. It's only been a few days since I disconnected the battery, but now that the weather is in the 20s I'll hope it climbs back up to the 26 or so I feel it gets in winter.

 

Locate ECM/ECU fuse, pull it out overnight.
That should fix it.

 

Car's computers are weird sometimes. If you really think the MPG dropped divide the miles you drove when you reset the trip odometer in your car every time you fill up normally by the amount of gallons that went into your car. I generally just think the car needs time to recalibrate itself properly. I feel like the MPG should go up normally but if you still feel like something is wrong after that maybe getting it checked out isn't a bad idea. I don't think anything would be wrong considering your car is fairly new though.