yeah me too, although ive just been starting to think about it a bit more.

my mate has an rs4 with 19's and the manufacturers recommend 34 psi

i have just picked up a bm 320 with 18's and the tyre pressures are set to 26.5 psi

and i run 19's on my cossie and not really sure what to set them at, or if the bm's are at the right pressure.