My winter ties are smaller than my summer tires

[88Racing]

Your gryphon oem tire size value is 2484mm w/6% squish. That's what I use and its verified using a gps. I have that same oem tire size.
A 285/50-20 would keep you in the same size.
A 265/50-20 would be 1/2"-3/4 less in height from the oem.
So about 2475mm for gryphon tire size value w/5.5% squish for the snows.
The gryphon uses milimeters instead of inches.
A 1/2" isn't that much to require a gear change.
The other thing is the oem 18" spare tire is the same size as the oem 20" tire.

Lars