According to my calculations 550/8 = 68.5 miles per hour. Even if you didn't take any breaks you had to slow down in some places due to traffic, tolls, ramps etc. From my experience to average 68.5 miles per hour you would have to drive steady 75 to 80 miles per hour most of the time and this is the type of speed where fuel efficiency starts to nose dive and probably that was main reason for such low mpg. I also found out, actually by accident, that in order to keep the heat inside the car constant the engine will run much more often. IMO poor cold weather fuel efficiency is due much more to heat generation than cold weather itself especially on shorter, slower, traffic light trips when running on battery and have the engine shut down is often possible. All in all 35 and 38 mpg is not bad at all. After all 38MPG is our old highway EPA rating and I'm sure the test is not done at 80 miles per hour in subfreezing temperatures.