I have a '17, bought 8/17, that has 17,000 miles on it. I have same tires that came with it. What's the deal on actual mileage vs the sheer time the tires are on the wheels re getting new tires? I'm not looking to spend $1000s ahead of time but don't want to ask for a highway problem either. Is it just a matter of tread thickness? I've heard there's more to it, regardless of mileage, if the tires have been in the car for, say, five years (as of 8/22).