The tyres are generally imagined as the last item, bought in and bolted on. Retailers list tyres by car brand as well as by dimension precisely because of this, since the correct answer for a given vehicle is frequently narrower than the size alone suggests. Owners who replace tyres without noticing the marking may end up with a product that is entirely legal, perfectly good and subtly different from what the car was engineered around. Staying with it preserves the characteristics the car was developed to have, which matters most on vehicles chosen for how they drive. Every significant system in a modern car has a similar story behind it, settled long before the first production body reaches the line.