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When I had my Insight, I found the difference in fuel economy between my summers (Potenza RE92) and winters (Nokian Nordman) was negligible. Tire size was identical. I typically ran the winters at 48PSI. There was a general decline in fuel economy, but it tracked outside temperature (which could be >80F between summer and winter), and there were no sudden changes around tire change time.
The Nordmans did a good enough job to drag the car through <6 inches of snow - I would only ever get stuck when I high centered the car. I could get good traction in ice slick enough I couldn't stand on it - I recall one time being unable to fully get up a hill, so I turned the car off and put it in 1st/ebrake, and got out, only to fall on my ass and slide down to the bottom of the hill. I couldn't get back up it to the car, parked halfway up the hill, without a rope.
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I have no need for winter tires where I live now, but on the topic of tires: On my MX-5, I'm thinking of swapping my Pilot Sports out for something much lower RR once they're finished. Outside of road trips I only drive the car maybe once a week, for pleasure, but premium is equivalent to ~$7.75US per gallon here (diesel is closer to $4.75 - I should have gotten a diesel) and over the last year and a half I've gotten most of the "go sideways" out of my system. I'm ready to get back into mostly eco-driving.
My current tires are 1.7% oversized and I make sure to calculate that in when I record a tank here.
I've personally never observed a steep decline in fuel economy with new tires. I wonder if the effect is more pronounced with wider tires?
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