Incorrect Tire Size = Differential or Tranny Problems?

[justben]

I won some wheels and tires on eBay for a good price. I was going to install them on my x5 01 3.0i, but they have 275/45-20 and 285/50-20 tires on the front and back. (they are 20x9 and 10.5 Asa wheels). I was hoping not to have to buy new tires immediately, but am concerned that using such big tires in the back will lead to serious transmission issues. I was going to get 275/40-20 and 295/40-20 -or- 315/35-20 in the rear when the time comes.

Should I install these wheels now or wait until I can get correct sized tires?