Advice needed grill blocking

Hi all. I’ve been driving a Prius Comfort 2006 for about half a year now. My average mileage during this period was about 56.1 MPG, actually pretty good. And above my expectations.
But now the winter has kicked in real hard and at the moment it is about -12 Celsius (approx. 10 Fahrenheit) and the display shows I’m currently doing 43 MPG and it's still dropping. So I wanted to try the grill blocking solution. I assume the first thing to do is blocking only the lower grill?

A little about the climate over here: It’s usually (10 months of year round) very mild, between 40 and 70 Fahrenheit with a few exceptions: we occasionally have 2-3 weeks in summer over 90 F and in wintertime a few weeks of extreme cold down to 5 F maximum. But these extremes are very rare and if they happen they last for only a couple of weeks.

When it gets a little warmer I will de-clutter my garage (used as storage room right now) and use it next winter.
Should I start blocking the lower grill and see if it makes any improvement in my mileage? And can I leave it blocked, even in summer when temperatures exceed 80 F ?

And what about the top grill, leave it open or only block it during extreme cold.

Hope anyone can point me in the right direction. I would be very gratefull.

P.S. Also heard something about changing (or preheating) the coolant to improve fuel economy, couldn’t find the exact way to do that.

 

Thanks you Ksstathead, I ussume you mean: switching the percentages of the upper and lower blockages. Because on the G2 the invertor/radiator is located at the top.

So like right now, way below 41 F I should block the upper grill fully (100%) and the lower grill 50%. Do I understand that correctly?

Thanks for your reply!

 

Thanks Tom, a lot of usefull information and links there. Hear the weather in the US is pretty bad as well.
Greetings from across the pond, hope to visit NYC again soon

Floyd