ChicagoSilver

I was only able to do 34 mpg on my comute this morning I had the heater on for half the trip where normally going to work I can pull between 41-44 its Cold in Chicago about 26 degrees, I have a question the best mpg on the hwy Ive been able to get is 38 mpg and that is with eco cruise at 70 mph over a seven hour trip from Chicago to Youngstown Ohio, what am I doing wrong I figure I would be mid 40s.