in the Spec subsite -- if, for instance, both sizes happen to be rated "88" by the European system of load ratings, then you might want to try the same pressures as the filler lid specifies (although I'm always happier with the handling with 3 or 4 pounds more pressure), but if your new tires have a lower E-code load number, you definitely need more pressure. In general, lower profile tires may be designed for a bit higher pressures with the same weight, though.