Which is one of the main reasons I bought one. I could keep tanks in the 38-42 MPG range, but my wife would drag the averages down as of late into the 33-35 MPG range. With the Prius, I'm hoping to get the average more in the 55 MPG if not better.
I believe one of the biggest reasons cold wx means a drop in MPG is the engine will run a whole lot more to generate enough heat to heat the interior of the car. Grill blocking will of course help hold the heat in the engine and the engine won't have to operate so much to generate heat for the car.
Oh and way back around 86 or so my VW Golf Diesel regularly gave at least 55 MPG highway and 35 or so in town
Winter fuel blends are less dense, when they have to be more volatile to allow cold weather starting. The same volatility in hot weather would cause excessive evaporative emissions and vapor lock. Also, more dense air means greater part-throttle pumping loss in traditional engines.
