If traffic was steady but slow, like ~40 miles per hour (in my experience, and sorry too early to convert ), I think you can get some pretty impressive mileage as you slip in and out of EV mode, coast, etc. That'd be my suggestion for the primary reason. Though when it's warmed up already before you start tracking (ie, fill up after it's warm) you do lose the initial warmup gas burn which I figure is a major part of why mileage is lower usually--the rest of the trip is compensating for all of that gas lost while getting the engine warmed up.