GM posi tire size differance?

[130fe]

I am wondering if anyone knows what the allowable tire diameter differance is. I have 275/40/17 (25.66 in) on mine and am wanting to use a either a C5/4 spare with a 25.54 (C4) or 24.99 (New GTO). Any help would be appreciated.

[shep]

I would guess that the vette spare would be the better choice I don't think a .12 of an inch will throw off the posi. I think that .6 inches could cause problems. But I could be wrong about that.