For those in the Northern Plains of the U.S. and in middle-Canada, I sympathize.
RH77 reporting on-location in Sioux City, Iowa. 
Tonight, the low is expected to dip to -15F, which makes vehicle operation, "interesting" -- and definitely not efficient. The vehicle I was assigned is equipped with a Continuously Variable Transmission. When it warmed-up to +10F today, it was able to operate efficiently, but from a cold soak at 0-deg F, it has a tough time. Better gloves were required.
From a cold-start and go, the CVT keeps revs high, and makes it very inefficient; however, once warmed, driving at a variety of speeds from 30-70 MPH, RPMS are 1300-2000. This is the key to it's efficiency. When it was first started, I thought the parking brake was stuck, but that's the nature of the super-cold CVT. Driving slowly to warm-up didn't translate into low RPMs.
More on the vehicle later, the Nissan Rogue 2.5 S AWD is CVT only and is the more-efficient contender in this small-SUV/Crossover class (based on the Sentra). SG results will be inaccurate due to temps and shorter distances, but an idea can be gained.
I'm told that this cold-snap has been consistent and relentless up North. Has this effected your FE?
RH77