[kwren]

Sorry I don't know personaly,
but CrazyPilot says:
On an 18x7.5 wheel I would go at least 225 width. 7.5 is the stock wheel width, and most SVXii run 225/50/16 on those. I have 205 width snow tires on my stock wheels, and they look noticably too narrow for the wheel. I don't care for snow though, because I want a narrow tire for winter.

My suggestions:

1. 225/40/18
2. 235/40/18
3. 235/35/18
4. 245/35/18

In my opinion 1 and 2 are the best size for a 7.5" rim. 3 and 4 are best suited to an 8" wide rim. #1 keeps the closest match to the stock diameter, but all are well within the 3% difference considered the limit.

245 is a wide as I would go. If you look at tire fitment charts (I'm a licensed tire technician, so this is stuff I do every day), you'll see that 255 is best suited to rims 8.5-10" wide. I can make tires far out of the fitment range work, but it's not a great idea.

Maybe 235 0r 245 for your wider rims?
Good luck, Keith