Originally Posted by doctacarta
Resolution: restarting the ECU and being told by my local Subaru technician that gas manufacturers lower the volatility in winter gas which especially effects the MPG in my particular car. Case closed!
That, and if you live in colder climates, warming up your engine will lower mpg, too. I was averaging around 22-23 mpg in the summer and now I'm around 19-20 (@13% drop).