It probably isn't a function of the size of the tire as much as the type of tire itself. The Fortera tire seems to be a "jack of all trades" tire, with no specific strength or weakness. Switching to a tire with more focused winter performance (maybe even dedicated snow tire) will gain you the most traction in winter weather. Going from the 20" to an 18" will help the tire grab a little better because a taller sidewall can conform to countours of the road easier, but that change is minimal.I just drove my '14 Diesel Overland with the OEM 20" Fortera M&S tires, up to our house in the mountains through sleet, snow and freezing rain. It does seem like my '07 limited with 18s was maybe a bit stickier in the slush. Do 18s really give you more traction in the slop ? And why ?