How important is the tire speed rating?I have a 2011 Chevy Express LT 3500 2WD that I carry my six kids around in (ages 10-2) and we do not tow anything. The original tires are LT 245/75r16 load index 116/120 speed rating S. The tire dealer wanted to put speed rating R tires on the van. Everything else is the same as the original tires except for the speed rating. Is this ok? Thanks! Eric