The big omission on that site is that it fails to take into account the effect of rim width on the other dimensions. Both the section width and the sidewall height will change as the same tire is mounted on rims of different widths. The difference in section width will be about 40 percent of the difference in rim width, so if a 215/50 tire is mounted on a rim one inch (equals 25.4.mm) narrower than the measuring rim, the mounted tire' section width will be about 10 mm less than what is molded into the sidewall. In addition, the sidewall will be elongated (depending on the curvature and stiffness of the sidewall in the original design) by a smaller, indeterminate-until-actually-measured amount. Moreover, because the aspect ratio is the sidewall height divided by the section width, having a smaller section width and larger sidewall height will make for a larger aspect ratio. On a narrower rim than the measuring rim, a tire marked 215/50 could actually be a 205/55.Very cool website indeed.
Now I feel comfortable to get some 18x7.5 with +48 offset wheels and 225/40R18 tires on my speed6.
The stock wheels have +50 offset, right?