Actually they have the same offset as the factory 15x7's (+6), so even though the backspacing changes the center line of the rim stays exactly the same. The 15x6's lose a 1/2" of rim on both the inside and the outside compared to the 15x7's.
The only difference might be that because the 15x6's are the smallest rim approved for a 225/60/15, the 225's might be a little more squeezed on the 15x6's than on the 15x7's. So a little clearance might be gained. It would be a fairly small difference though.
