Im looking to replace my 255/ 35/18 rear tires with 245/40/18 tires. The tires are bald and need replacing so while I'm at it, I figure I would get slightly smaller tires for the rear. My question is, if I go to 245/40/18 What is the difference between 35 and 40? Anything major? Does that number measure width? Clueless here.