The 17X8.5 ET55 with the 255/40R17 can rub.

 

[QUOTE=jeffbrig,May 19 2009, 01:55 PM] I've never liked this table from the FAQ.

 

Yep that helps thanks!

 

Another one...

Rims don't rub...tires do.

The offset determines where the center of the tire is located, just like it determines where the center of the wheel is located. So if you know where the center of the tire tread is located and how wide the tire is, you can predict if it will rub or not.

With only 40mm offset, you won't be able to run much over a 235 without rubbing. That is about as small a tire as you want to run on an 8" rim. You might get away with a 245 but it would depend on the tire.

If you get 255s for the back on 40mm offset rims and they don't rub, I will delete both my posts in this thread and eat my hat.

 

But...tire dimensions are influenced to some extent by the width of the rim. Ever notice that tire specs give a range of wheel widths it will mount to, but then list the specific rim size used to measure the full specs? Mount a given tire on a 7" rim, and you're going to pinch it to a narrower width. Mount the same tire on a 9" rim and it's going to pull it wider. The corner profile as well as the contact patch width are affected by this stretching. It won't be the full 2" difference in the wheel size, but it will certainly be a measurable difference, and it can affect fender clearance.


That said, tof is right, the wheel offset will be the primary variable in locating the center of the tire. A narrow wheel will pinch the tire a little, but it won't be a 1:1 ratio changing the width of the tire at the point where it can rub. So, that 6mm gain from the narrower rim I was predicting above is going to be less than 6mm.

(tof, thanks for helping my understanding on that)

 

OK, but lets remember that the point where a tire rubs is USUALLY around the outside tread edge. That point doesn't vary as much with rim width as other sections of the sidewall. And the variation in sidewall width from one rim-width to another is still much less than the variation from one tire size to another (10 MM increments). In fact it is probably less than the variation between different brands and models of tire. So the tread width rating is still the only useful wheel/tire parameter in determining if a given setup will rub or not (again, assuming you don't go nuts and put something with a larger diameter like a 245/55 on the back.)

(jeffbrig, thanks for saying I was right about something. I don't get that very often )

ps: and always listen to Jim from TheTireRack because he is one pro who knows his s**t.