how much fuel mileage is average during the cold winter months?

0.2-0.3mpg

 

I'd guess at least 10% or more, depending on how much they thin the fuel, and how much more you idle.

 

I've looked at my IFTA mpg per month for the last three years.

July and September are my best months, 9.5 mpg avg (or higher. Swift caps the mpg at 9.5). April is my worst month at 8.0 mpg avg. It looks like the seasonal variability is about 20%.

Rain, wind and temp are the three major factors for mpg, however, it always feels like I lose mpg's for no reason in the fall (I see them start to drop before the weather turns) followed by a gain in the spring (mpg's start to improve before the weather really does).

I do most of my driving on I5 going between Sumner WA and Yreka CA. With the loads and returns always about the same weight. My idle times remain about the same throughout the year. It takes the same amount of time to make a pot of coffee in the summer as it does during the winter.