HI!

The answer to your question is that you are supposed to choose a tire which has the same sidewall stiffness as the original tire. Higher speed rated/performance rated tires have a stiffer sidewall to generate less flexing while cornering, accelerating and braking. If you decide to go with a cheaper tire which has a lower rating, eg. from a z rated to a T rated, then the tire is going to need more pressure to make up for the softer sidewall..

How do you think they make 'run flats'? They just have an extremely stiff sidewall, and are usually mounted on high performance cars. :-))