saharv23

I've had 2 Sentras (91 SE traded w/158k, 97 GXE still driving w/183k) and both have acted the same. I've learned to expect it, but still am curious. Can anyone explain, please?
I drive 120 highway miles/day commuting. Every fall when cold weather comes to Iowa, my gas mileage drops about 10%. From 38mpg to about 35. When spring comes it goes back up. If I go south for a winter vacation, the mileage goes up, then drops off when I return home. Both cars have done this since they were new. Is this the cars response to the cold air? Is it a result of winter formula gas? BTW I use 10% ethanol, unless I can't find it. Thanks. P.S. I don't idle the car to warm it up during cold weather.
Steve
southiowa