Run 4, regardless of vehicle. That way you maintain equal grip for braking and cornering front and rear. Plus you can rotate them next winter. With that said, I am running 4 studded tires on my 2011 3rd gen and still avgerage mid 40's. SPH-D710 ? 2
I can't really tell how it influenced my mpg because it also started getting cold. My average difference between warm & cold weather is about 6% more. It doesn't get very cold here so the difference in colder climates would be even more. ⛄
Sliding into the ditch gets you zero MPG. Do it right and get a proper set of winter tires. The combination of winter blend fuel and freezing temps have proven to be way more significant than my winter tires in terms of fuel economy.
Ditto here... always 4 snow tires. Don't want to loop the rear end of your car around every corner. Not fun. For the best MPG in Winter... drive extra gently, don't store stuff in the car (keep it light) and check you tire inflation pressure on regular intervals. 10 degrees of temperature drop will lower tire pressure about 1 psi. When looking for snows, make sure you go with the narrowest recommended.
Four snow tires. Anyone and everyone will tell you that. If I'm not mistaken, in some areas police will tell you that too, formally. As far as mpg, I found it's a wash, between our stock Michelin Pilots in 215/45R17, and (4, on steel rims) Michelin X-Ice2 in 195/65R15. What a difference a decade makes though: go back a few years, well 30 years I guess, and two snow tires was the norm. I remember coming down a snowy hill with 2 snows on the front of a front wheel drive Civic, feeling the back end starting to come loose. Granted any combo will lose traction eventually, but having snows all 'round can make the difference. Plus, with all 4 stock tires off, it's a good opportunity to detail them all, and keeps them out of winter conditions.