There are lots of factors that decrease winter fuel economy. If you are like me, you start your engine & then go scrape ice or clean off snow. You get zero MPG while idleing & not moving. Cold engine oil, tranny oil etc increase drag until they warm up, & as you say, accidents & other problems increase time spent with engine running & no movement. The extra drag from using 4WD also factors in, but the mileage drops even in a 2wd rig.

I've been running both for over 20 years up here & I bet I could tell you more or less when the major storms came in by looking at the mileage logs.

Dave