BMW X5 comes fitted with run flat type tyres and that's what BMW recommends. In 255/55R18 we have Kumho K51 tyres but it has H(210) speed rating. I will recommend you to go for tyres with V speed rating. Please look for various tyre options from here- http://tyres.cardekho.com/search-tyres. If you don't want to go for run flat tyre then I will suggest you Pirelli Scorpion Verde tyre. However remember to keep emergency inflator or BMW's space saver Spare wheel in the car.