[Bill Verburg]

Originally Posted by JJ 911SC

I'm going from 7.5" rims to 7" and wanted to know the impact on the tire measurement.

Came across this site, very useful; Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Enjoy

a wider wheel will do two things to a tire
1) stretch the overall width by ~.2" per additional .5" of wheel width note that the tread width is not altered by a change in wheel width
2) reduce the unloaded overall diameter of the tire by ~ .1" per additional .5" of wheel width.

So for your change from a 7 to a 7.5" wheel w/ a nominal 205/50 x17 tire expect the following change. I chose a BS RE-11 tire as typical

base measurements for this tire are on a 6.5" wheel
section width = 8.4"
overall diameter = 25.1"
the tire is distorted into a 213/48 x17

this tire mounted on a 7" wheel can be expected to measure
section width = 8.6"
overall diameter = 25.0"
the tire is distorted into a 218/47 x17

on a 7.5" wheel
section width = 8.8"
overall diameter = 24.9"
the tire is distorted into a 223/46 x17