You can calculate the dimensions quite easily without chasing a tire site.

Lets use a 285-35-18 as an example.

The 285 part is the width, in millimeters. 1 inch = 25.4mm. So if we divide 285/25.4, the width in inches is 11.22".

The 35 part is aspect ratio, which is the height of the sidewall, in percent of the tire width. So, 11.22x.35 = 3.927", which is the height of the sidewall.

Now, for the overall diameter, we need 2 x 3.927 (for two sidewall heights) + 18" for the wheel height. 7.854" + 18" = 25.854" total tire diameter.

You can set this up in a spreadsheet in a few minutes, and then get results, or a table, in seconds.

Here are some numbers I did the other day, looking at the stock S4 rims/tires, compared to the Saturn v8 wheels/tires. The stock Saturn rear, is the 285 one. Goodyear no longer makes much of anything in 285/35/18, so I am considering going with a 275/35/18, that are still available, at a good price.

Rears

245-45-17 9.65 25.68 stock S4

275-35-18 10.83 25.58

285-35-18 11.22 25.85 stock Saturn

295-35-18 11.61 26.13

Fronts

215-40-17 8.46 23.77 stock S4

235-40-17 9.25 24.40 stock Saturn

numbers updated 7/15

Brian