I've got a set of barely used snow tires from an old vehicle in the size of 205/55r16. The size on my '11 is 215/60r16, so obviously there's a difference in diameter. Other than looks, I can live with the increase in wheel well gap and lower ride height for 4 months, and the change in MPH/ODO readings would there be any safety or sensor (ABS) issues?
You will be dropping 10 mm in width, and about 17 mm in height, which is substantial. The main issue will be speedometer will be off, I don't think there would be any other issues
Stock 82.1 inch circumference compared to your proposed 78.1 circumference equals a 5% difference. Traveling at an indicated 60 mph will be an actual 57 mph. Most reputable tire shops will let you get by with a 2% difference.
But more importantly your proposal will loose you more than 5/8" ground clearance at the time of year you need it most.
You may have saved money on the wrong size of tires, but it may cause you to damage something under your car while driving through snow or ice, or lead you to being stuck and requiring a tow. Either way it will cost you.