So, "c"'s MPG is more affected by cold?

Depending on my route, speed, and ambient temp I may block with anywhere from 1 to 4 long strips of pipe insulation. I never block the last (lowest) of the grille opening slots.
BUT, I believe it's vital to have a means of monitoring coolant temp before doing any grille blocking.
Coolant temp will change dramatically when the heater is off (set to "Lo") or set to warm the cabin.

 

The C seems to work just like the GenII Prius. Different than the GenIII hatchback, that's just from personal experience.
The C seems

 

"I've looked at your Fuelly log and it looks your first winter was much worse than this one. Did you block it back then? or was it due to break-in?"

I can't find how to view more than 30 fill-ups on Fuelly (anyone know how?) but I was grille blocking before 2/2/14.
I'd guess the relatively poor winter mileage last year was due to (1) Break-in, (2) more 400 mile highway trips to VT and back.

 

At the bottom of the Log Book section there are Next and Previous buttons that let you view all the entries one screen at a time. And there's [View All Fuel-ups] which shows all entries on one screen but in an expanded format.
Edit: There's also an option to export all data to a CSV file, which can then be imported into a spreadsheet or database on your PC. This is at the bottom of the Log Book section, disguised as a little blue disk icon:

 

"At the bottom of the Log Book section there are Next and Previous buttons..."

Thanks, don't know how I missed that. I started grille blocking on 12/21/13, so a bit later in the season than this year.

 

There's no benefit to blocking the grill unless your coolant thermostat is failing to keep the engine up to operating temperature. For cars with electric fans, leave a gap between the blocking material and the radiator, so the fan can pull air through the rad if it requires it. There may be some aerodynamic benefit from closing-off the grill-openings, but I doubt it would be measurable.

 

cold engines will get less mpg simply because the cold air and engine need more fuel. blocking off the radiator will help the engine warm up but the longer it takes the less mpg. true of all IC engines. cold air only helps at max power requirements because air is denser and for same reason when cold air is charged to combustion chamber more fuel is needed to burn properly.

 

actually there is. at really cold temps the radiator will cool the coolant so much thateven virtually closed thermostat will not let engine reach design operating temperature. and you don't have to live in AK for thsat. I've seen it in MI.

 

My c is very sensitive to cold. My commute is only 2.4 miles. I have all the grill blocked that seems to help a little on this short trip. On longer trips of over 10 miles my mpg will climb up close to 70. But most trips are short so average is only 52 this year so far.

 

Very little MPG is lost through the grill, except in very cold conditions. You are losing it mainly through the increased viscosity of the lubricants.

 

It fails to do that, except in very cold weather.

 

On my second winter with this car and I drive about 90% the same tank to tank.

In the winter I consistently drop 10mpg on the same route to work. It seems to come in steps; if the temperature is below freezing (as it is much of winter here) I lose 10mpg. If its between 32(f) and about 50(f) I lost around 5mpg. Anything over 60(f) and I can usually get better than 60mpg.

My route is exactly 5.4 miles, and I have never blocked my grill (though I should, I am afraid I would forget to remove it for long trips).