I'm about to buy Nokian WR G2s for my HCH II and have a question regarding speed ratings. Obviously, differently-rated tires have higher or lower speeds, depending on the rating. As for the different ratings, are the compounds they are constructed from different and would a lower or higher rated tire perform better from a fuel economy and LRR standpoint? Speed isn't a concern, but MPGs and winter handling are big issues up here in northern Illinois. Thank You!