The way I usually explain it to customers calling in asking about down sizing is going down in wheel size does a few things.
1) The wheel/tire combo usually is just a little more expensive then the stock winter sizes would be, especially when you talk about having someone unmounting tires from your stock wheels twice a year.
2) A smaller wheel means more rubber between the road and the wheel (overall diameter stays about the same). This is great to soak up all those pot holes in the winter and not damage you stock wheels.
3) Narrow tires will be better for cutting through deep snow and get you a better footprint on the road instead of floating on top of the snow.
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