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Took a trip from Chicago to St Louis last weekend and noticed some intersting data. Drove our 08 Pacifica (hauling 4 people, suitcases cooler and a dog) and got horrible milage on the way down. Note, the trip is mostly long stretches of flat highway with the only hills you find are overpasses.

On the way down the weather was cold with high winds and blowing snow for the first 200 miles. Rest of trip was OK. Car got 21 mpg on the trip down. Not many chances to hypermile as we slowed down for drifting snow and crashed cars. We sat still a number of times but with entire faimily could not shut down ICE. Its hard get better milage when you are just idling along behind the crowd!

Weather was good all the way home. Temperature when we left St Louis was 45 deg. The first 100 mile traveled with traffic cruzing at 65-68 mph. Car averaged 24 mpg. As we continued north, the temperature started to drop. The SG2 indicated a steady drop in mpg as the temperature dropped down to around 15 degrees.

I reset the SG2 for the last leg of the trip (still flat with few hills and kept same speed). The last 100 miles or so in 15 degree weather resulted in an average mpg of 23.2. A 4% drop in mpg just because of the outside temperature.

I am tired of winter and wish for Spring!!!!

I feel your pain. I lived in snow country for 15 years, and had plenty of driving experiences where my stomach was tied in knots. My IHC Scout 4X4 got about 11-12 mpg in the summer (with an all-time best tank of 16 mpg), but its winter mileage dropped to 5-6 mpg. Needless to say, we drove our Datsun B210 a lot more than the Scout.

Four wheel drive's advantage in the snow is an illusion. That 4500 lb. Scout with studded rear tires required twice the distance to stop in snow as the B210 with studded tires on all four corners. I only got the RWD B210 stuck in the snow three times, and we pushed it out easily twice. I had to put the chains on the third time.

Yup.

The colder the combustion chamber walls (head, valves, piston, block), the more fuel energy gets wasted in warming them up.

Numbers from my truck:

when fully warmed up (coolant around 190 F), my truck idles with ~ .3-.4 gal/hour
when I've just started up cold soaked to 20F or so, it takes more like 2 gal/hour

My last full tank was with the coldest weather we've had this winter, no grill block, and I got an all time low of 18.5 mpg.
The current tank is running more like 21.5mpg with the only change being ~ 20F warmer weather and the installation of the factory winter cover (full grill block). That works out to something like 1% fuel economy improvement for ever 1 deg F warmer. Now if only that trend would continue into summer..... (Just kidding, I know that at warmer temps the thermostat & fan controls operating temps more than the ambient air).

The grill block seems to help it warm up quicker and reach a higher steady-state temp (more like 190 instead of 175F). Both are good for mpg.