We were in Boston for 6 days over Christmas and drove my 2004 Prius up and back. The temps weren't quite as low, but for our in-town driving (up and down RT 3 and then some of the backroads), we averaged 45. For the actual highway driving from DC to Boston, about 50. One thing I noticed (aside from there being NO Priuses there--I actually got waved at!) was that in downtown Boston (near Chinatown and the Science Museum) the Navigation was not accurate (probably all the construction due to the Big Dig). If that is important to you, you might want to make sure to get an '06 and updated DVD. The navigation was GREAT for the whole way up and back, including going entirely back roads from Plymouth to Newport, RI.

--
Hillary