Running staggered tires

[Cobrajet]

Hey All,
So in Short 235/45/18 & 255/45/18
Will have different diameters.
I thought just the width would be different.
I just want to run a little bit wider in the rear then the front.

Definitely not the same diameter. Check a few different tire spec sheets on tirerack.com, and you can find out which tires will have the same diameter.

One I checked:
235/45/18 = 26.4"
255/45/18 = 26.7"

By comparison, the stock Bridgestone Dueler 245/60R18 is 29.7".