this is taken from tire tech
Note: The overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.
The industry rule of thumb is that for every 1/2″ change in rim width, the tire’s section width will correspondingly change by approximately 2/10″.
For example: a tire in the P205/60R15 size is measured on a 6.0″ wide wheel and this size tire has an approved rim width range from 5.5″ to 7.5″ wide. The tire has a section width of 8.23″ (209mm) when mounted on a 6.0″ wide wheel. If that tire were mounted on all of the rims within its approved range, the tire’s approximate section width would change as follows: