Question: what is the correct tire pressure for low profile tires?
Answer: The recommended pressures? For 225/65R17, it's 2.3 bar and for 235/55R18 it's only 2.2 bar. I suspect the pressure difference is due to the width of the low-profile tire. Usually, low-profile tires are wider, and therefore, you could use a bit lower pressure than for normal-profile tires.