Corolla MPG is so temperature-sensitive?

The fuel economy of my 2003 Corolla LE automatic seems to vary a lot through the year. In the summer it could reach 40MPG with a mixed driving of 75% highway and 25% local. However it could bottom around 30-32 MPG under the same driving condition when the temperature is around 20's F in the winter. I understand that it needs to burn more gas when it's cold, but such a huge fluctuation still surprises me, and I don't see such a big range in other brands. Is this normal?

Don't forget that the gas companies change fuel formulas throughout the season to comply with federal air quality standards. Winter blends will have more ethanol, which lowers economy by ~5% versus straight gasoline. Winter temps also decrease your tire pressure by 5-10%, adding rolling resistance. Add to that the winter routine of idling the car to warm it up and your gas mileage can suffer dramatically without you knowing it.

Bigger vehicles are less sensitive since they've got low MPGs anyways, but your little Corolla will easily show a drop when something changes. Hope this helps!

Might be the oil is thicker and it takes more energy to turn the engine, if you follow my drift. 20F isn't all that cold and you should be using a 5W-20 or maybe 0W-20 to let the engine turn easier than using a thicker viscosity oil. Great strides have been made in oil quality and in SC on the coast near Savannah, GA it gets extremely hot and the cops cars all use 5W-20 Ford synthetic blend serviced by the Ford dealer here. Unbelievable how they do extensive idling in 100F weather and never does damage to the engine. I have a new 2009 Coralla and it says use 0W-20 or 5W-20 but until I get say 25K mi on it I think I ask the Toyota place to use 0W-20.

Jdmccright is on target about tire pressure. I usually run 2lbs more than recommended but when I go on a trip I drop 2lbs wher it should be. My rationale is I do most city and rural driving not like on the Interstate where your tires get real hot and running 2lbs over won't hurt anything. I just saw on FoxNews where the Dept of Energy says the price of gas will remain at $4.00 a gal or higher through 2009. Well, .. . ddduuuhhh! Tell me something I didn't know . . .

By the way, my Corolla automatic tranny with barely 500 mi on it and I'm getting 30 mpg city and haven't done a road trip with it yet. Happy, happy happy with that.