Lower highway mileage?
 

I average 40-43 when commuting to work on weekdays with weekend driving mixed in, but whenever I get on the interstate for a road trip my mileage falls into the 30s. My two trips to Atlanta and to Lafayette are my only tanks in the thirties so far, both 37. I drive at 65mph to keep from being run over.

You are probably stuck in fast moving traffic during the week and end up drafting without realizing it and then on the long trips you get into the wide open spaces and have to push all that air out of the way yourself.

Ditto what JanGeo said. I find I do better commuting during rush hour. During off hours I can drive slower, but I have to plow my own hole through the air.