I run 255/55/18 BF Goodrich All Terrain T/A KO2 tyres on my XC90 and they're brilliant. I have always subcribed to the theory of buying the best tyres I can afford at any given time. Good tyres can make up for a lot. I've seen a 2wd car with good off road tyres make it up hills that 'proper' 4x4s with street tyres fail totally. And, as others have mentioned, running good all terrain tyres has other benefits as well, such as strength and longevity. I've been running BFG tyres on many different cars for many years now, and usually get at least 50k miles out of them, and have NEVER had a puncture or other failure of any kind. You get what you pay for. Besides, chunky tyres on an XC90 look good!