So it turns out that the full sized spare wheel does not fit in to the wheel well at all, and sticks up from the boot floor around six inches. This is more boot space than I am prepared to give up. Also, the Subaru dealer said that they were unable to find a bolt long enough to secure it to what the space saver spare wheel was bolted to. So I will give up on this idea for now and keep the space saver spare wheel. I will have the money I paid for the full sized wheel refunded. What I don't understand is that the space saver spare wheel must be the same diameter as the wheels on the car, only slimmer? So why won't a full sized spare wheel go some of the way in to the spare wheel well? I was prepared to make something out of wood to raise the boot floor around 2 inches, but 6 inches is too much. The Subaru dealer said that it might be possible to get a steel spare wheel that would fit. That he thought would be better than the space saver. However, there would still be the 50 mile an hour speed restriction for this wheel. The XT has 18 inch diameter wheels, but I don't see why this would make any difference.