If this was only a "one-way" portion of trip it was probably two things:

Your lower speeds. Slower speeds really help. (as in 60. 70 is too high)
A tail wind.

And perhaps a little bit of drafting if traffic was that heavy.


My best "longer" trips were:

Madison WI - Chicago (NW suburbs - Arlington Heights). Returning home from HybridFest 2006. 125 miles. 48 MPG.

Detroit (New Hudson NW suburb) - Chicago (Arlington Heights). Last weekend. 300 miles. 47 MPG.

Normally 43 MPG is pretty easy (at around 60-65 miles per hour). On any length of highway trip.