Driver72 you do get it and I will respond to you. I understand the offset/backset. I did not work in a tire shop and instead received an engineering degree, with a math minor. The poster above is correct the math is right. He is wrong when he says no need to check however. I was also a carpenter and now direct large construction. Check and re-check your measurements!
Now one more time. This concerns the backs only. I would not mess with the front, go for some 9.5's and leave the spacing alone. Don't get me wrong, if we could stuff the wheels and tires back in there 40 mm I would be happy, but there is a hard problem we can't move. It is located on the front inside. It looks like a rib in the liner. Coldheartid reports his large 315's on the 11" with ONLY a 21mm offset is close. He posted he can run his finger btw the tire and the hard surface.
1. I have 305/35 mounted on the OEM 9.5's. I can move a 1/2" rod between the hard surface and the TIRE.
2. I posted pictures for everyone showing 1- edge of a 305 on the outside is dead even with the sheet metal on the top. And 2- the 9.5" wheel edge is 5/8" inside of the outside edges of the 305, it will be a bit worse or greater with a 315. Now I don't care if you want to be anal with the outside edge of the wider wheels but the tire really conceals it. Besides the problem is that when you talk about offset you are moving the centerline of the TIRE/wheel. In this case we can only move that tire/wheel assembly so far before it will rub the inside.
3. Considering ColdheartId comments and his wonderful job of pioneering and finding nearly the only off the shelf wheels that work along with my measurements AND I don't give a damn about where the wheel edge sits. I think we can order a 25mm offset with some 305's and they won't rub. I am not ready to say another 5mm or 1/4" (30mm) works. I really have my doubts you can fit a 315 with a 30 mm offset. You may get a 305 at 30 mm, but I will let someone else try it for me.
I want to be wrong on the prediction. I am not wrong on the relationships and offset/backset. Anyone who is selecting their own equipment better be able to read those fundamental dimension charts like I posted yesterday and not just pick offset numbers because they are even numbers unless your retailer has a generous return policy.