Rear Versus Front Tire Widths

Ok so just to get the terminology squared away;
Everybody I know, when talking about tires, never mentions the tread size. But, the profile width is an ethereal number, that is only accurate when the tire is installed on the checking wheel, on which it was installed, at the tire factory.

Now, when the tire company designs a 215 tire, it will only be a 215 on a checking wheel that is equal to 70% of the branded 215 size. Since a 215, converts to 215/25.4= 8.46 inches, and 70% of that is 5.925, which rounds to a 6" wheel.
Therefore, when you mount a 215, on a 6" wheel, it should have a profile width of 215 +/- 5mm.
Now then,
If you mount a 215 on a 7"wheel, the profile will be about half the wheel-difference larger . so then, your 215 become 215 plus half of one inch(an inch being 25.4mm), equals 227.5mm, +/- 5mm

I just thought that you should know this ... in case you didn't.