235's or 30's?

I'm planning on purchasing a set of BFG At's before the snow falls. I've been planning on getting 235/75/15 but my friend says I should go with 30's. I've got a 00 XJ with a 4.0 and auto. I'm pretty sure the rear end is 3.73

1. Will I notice much of a difference in fuel economy going from the

1. Can I still use my original spare if I go with 235 or 30? I know I should have a full size spare but I don't do much offroading and right now I can't afford an extra tire.
2. Can I fit one of the above sizes, on a steel rim in the stock spare location, inside the back or will I need to mount it on the exterior?

Thanks in advance

Mort