Wider rubber is different to overall diameter.

If I have staggered setup,
Widder rims on rear, will obviously have wider rubber, but would be the same diameter as the front.

Ie: same diameter all round
The rear having wider rubber should'nt effect the rolling diameter and therefore shouldn't have any effect on the diffs.

Cornering at slow speeds may affect diffs, but hey it dose already, sometimes a rear will skid a little In carparks and that, but so what, it's an STi.

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my05 STi
my13 Rok

:)