The index rating on a tire basically reflects the speed limit that a tire can sustain for at least (IIRC) 10 mins before giving up and being destroyed, minus one. So a tire that sustained 10 min of U rating (200kph) speed and suffered drastic damage (typically, tire blows and tread band goes off) right after that mark gets a T rating (190kph). This is oversimplified but that's the gist of it.

You can thus see why the index of stock tires always vastly overshoot the top speed of a car, e.g my car gets a Y tyre stock even though its top speed is 245kph, and my ex car has a W rating when the top speed is 210kph.

It's not a good idea to go anywhere near the limit of your tire speed index, nor to go cheap and skimp on the index when buying a replacement tire.

I bet the Macan that gets 18" and the W index is the one with low enough top speeds (below 230kph, that'd be 2.0L four-pot petrol and 3.0L V6 Diesel ones)

EDIT: found a reference again WRT index tests

> Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=...