Lots of us has to use winter tires during the cold season, if not - one simply are not beeing able to get anywhere, as summer and all year tyres has to little grip in snow and on ice.
Winter tires are made of a softer compund, and has lots of small groves in the rubber. Below 7 degrees centigrade ordinary tyres starts to loose grip as the compund stiffens.
But I suspect, that the added grip from the wintertyres (soft compund and many grooves) adds rolling resistance?
On bare tarmac my winter tyres "sing" - I take this as a sign of added rolling resistance.
PS: Cant wait until spring when I can mount my LRR Michelin Energy Savers again
