Recommended tyre pressures 987.1/ 987.2 why so different?

[cslwannabe]

I own a 987.1 3.4S with 18" wheels and the recommended tyre pressure for the rears is 36 or 37 psi (I think the door sticker says 37 and the manual says 36), but noticed when I was reading an online manual for the 987.2, the recommended rear tyre pressure is 31 psi for 18" wheels.

Just curious as to why there is such a difference - anyone know why?

[20DHR]

I have 19 inch sports wheels
Tyres 235 fronts and265 rear
There's 5lbs difference
Seems high