Hi,I came across a fantastic deal on craigslist. I bought a brand new tire and wheel for my Mercedes for $50. The only thing is that it's a 16" and my other three are 15". I can't really notice a size difference when it's on the car. I mounted it on the driver's side rear. It's a directional rotating tire. Should I be concerned with the size difference and will it throw off the speedometer? Thanks.