225/40/18

[Wally]

I think complete books have been written on that subject and complete R&D departments are dedicated finding the 'right' combination...

I' am sure SteveC will have something smart to say about this though

My personal take is that the springs are there to provide the correct stiffness for a given acceptable body movement and the shocks are only there to regulate the rebounce of the springs.
Stiffer spring will mean a fitting shock setting, not the other way around:
When your shocks are going to take over the spring's task, your on the wrong path...which is seen quite often as shocks can sometimes be adjusted but springs cannot, so some people tend to make the shocks stiffer but actually wanted something that the springs are supposed to do for you like less overhang, less diving under braking etc. The resultant stiffer shock setting makes it a bumpy ride with worse handling instead of what was intended...
Adjustable shocks are there to adjust to your spring to, not to make a stiffer suspension!!
Just my personal 2 cents on the subject and way off topic on the subject of low profile tires...