Sorry to ask, but I am a novice.

What is the max tyre width on a 18 x 8 rim?

Thanks from the UK.

 

If you've got a tire in mind, you can look up the approved rim widths on www.tirerack.com.

245/40-18 looks reasonable (from Bridgestone S-03 specs).

 

Profile changes things a little, but I have 255s on the rear of my car and they really should not have been put on, oh well. Stick with a 225-245 IMO.

 

Yes, 225/45-18 or 245/40-18 should work.

For the street I'd use the 245/40 if for no other reason than they might protect your rims a LITTLE from LIGHT brushing against a curb. They'll probably lok better too.

The 225/45-18 might actually handle better on the front wheels, but I wouldn't use them at the rear and mixing would eliminate the option of rotating the tires.

Besides, with the 18" rims you're going to have very little sidewall compliance anyway, so the extra sidewall flex of the 245/40 may actually improve grip a bit.