Here is my observation re: winter MPG. My trip MPG doesn't suffer noticeably in my very predictable 10 mile city commute until ambient temps slip under 60F. My normal MPG is 60MPG (indicated), goes near 55MPG 40-60F and 50MPG under 40F. However, if I keep the car in attached, insulated garage overnight and the starting temp is near 60, there appears to be no MPG penalty even if the ambient temp is near 40F. Keep in mind, that I use no cabin heating at these mild temps, just a coat. Based on that, I suspect engine block heater and not grill blocking is the way to go to improve MPG in winter.
MPG suffer depends on battery temperature and engine heat-up time. Recently I filled-up car a day after temperatures dropped below freezing. Somewhere between -12 and -4 Celsius (10 - 24F). After about 300 km (187 mi) I refilled the car. Result: 6.9 l/100km (34 MPG) while normal MPG is about 47 - 50. I'm not using any block heater and the car stays outside all the time. In mild temperatures, just over freezing not using cabin heater improves MPG just like in your observation. But if battery goes below freezing enabling heating gives positive result. While battery is cold even slow crawling in traffic jam requires ICE working, no battery power at all. After it preheats then works fine. My commute is very short - only 10km (6.2 mi) one way. Most in traffic jam. EU Prius is different from US Prius in one thing. It will shut down engine in S2 state.
Good point. I forgot the battery temp issue. Looks like heated garage is the best solution. I'm glad I at least insulated the garage last spring.
Grill blocking will limit the amount of cold air that hits your radiator and engine, allowing the engine to warm up more quickly. As a consequence of this, you will hit the most efficient S4 state (159F) sooner. Also, once above 159F, you can turn on the heater and warm up the cabin (and the battery) which should help further still.
You will always have a mpg penalty in lower temperatures. The colder the engine/battery the worse the hit will be but beyond that you still have to cope with higher rolling resistance of the tires and road surface. If driving at high speeds then you also have to overcome increased air density thus increased drag.
I still think the engine/battery temp impact is huge compared to the tire/air impact (if one adjusts air pressure in cold temps).
I was focusing on rolling resistance due to reduced pliability of tire rubber and other components as well as road surfaces. Tests have shown that rolling resistance can almost double when temps drop from 100F to 32F (approx.). This is independent of air pressure and assume the same pressure in both temperature extemes. More on rolling resistance. Tire Review - Rolling Resistance: What Really Makes a Tire Fuel-Efficient? http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf
