I have been spending time on some of the wheel size charts to try and determine the right size wheels for my xB. The ones that I am using do not seem to take into account the width of the rim that the tire will go on to calculate the overall diameter. For example the charts I use indicate that a 215/60/15 tire has a diameter of 25.16 inches. However, if I put this tire on a 8" wide rim and get more stretch then a say a 7", will the overall diameter also decrease? Are there any calculators that take this into account?

Excellent question. I too have been wondering this.

My initial thought would be yes. Becuause by streatching the tire, you're reducing the height of the sidewall.

VW owners use this high-offset/streatched-tire look a lot. I wonder if www.vwvortex.com would have any info on this …

makes sense to me...if u stretch the tire and make the sidewalls sit at an angle...it would sound to me like u have pulled the tread/tire closer to the rim and making the overall diameter smaller....if u know the measurement of what it should be according to the chart.....then just measure your stretched tires and then compare....
post back here how much of a difference when u figure it out...would be neat to know

stretching the tires will decrease the diameter measured from the sidewall (if any) but the overall tires diameter shouldnt be effected too much to notice. Strething 215/60/15 on to a 8" wheel is not much of a stretch, so if you are asking for this specific example, i would say no, the diameter is not effected.

edit.. i have some 15x8s w/ 195/40-15s on them, and the stretch is hardly noticeable.

No calculator I've seen for that.
Insignificant amount anyway. Tires between manufacturers probably vary more than the minimal decrease that could be caused by a severe stretch. Remember, the charts are only approximate.

Here a tire caluclator with width added to the calucualtions. I got this from a earlier post and saved it on my favorites. It is giving me different results now that the calcualtions include the wheel width instead of the tire calculators that omit the wheel width. Try it out.

http://gs.tolan-hoechst.com/tirecalc.htm

Don't think so. Give me an example where the same tire with different wheel width gives a different overall diameter on that calculator.