As you can see from the attached photos my new to me Boxster S has custom wheels.
How do I know what pressure to inflate the tyres to as obviously the chart in the door jam is for the standard wheels?

 

All tires (at least in the US?) have the pressure rating scribed into the sidewall of the tire.

 

Isn't that just the maximum pressure? The running pressure would vary car to car?

 

Dude. The car doesn't know what kind of wheels are on your car. Nor does it care. What are you trying to achieve?

Just put 28 in the front and 32 in the rear.

 

Watch this...it explains why you can't use necessarily the same psi when you change wheel or tyre size.

 

... I always fill my tires almost up to the max pressure... never had a problem on the road...

Is this a track car?

 

No...just new to me!
I'm probably just overthinking it!

 

I overthought when I first got my car. Lasted for more than a few years. It'll wear off... and then you can just hop in, start 'er up, and drive. You'll recover, lol.

 

Hahaha👍