If you want 20's, the best fitment is 245/40/20 in front, and 275/35/20 in rear. The only problem is no one makes runflats in this size. If runflats are what you are looking for with 20's, you will need 245/35/20 in front, 285/30/20 in rear, but these tires leave a lot of wheel well.
Front = 20X8.5 +38mm offset
Rear = 20X9 or 20X9.5 +47-49mm offset
The rear is a hard fitment for 20s. After 4 trys with Asanti AF-135 chrome, the best fitment without the tires rubbing in any way (275/35/20) is 20X9 with +49 offset.
After market wheels look great on the XLR-V, and I seem to have a lot more response and questions with these wheels on compared to the factory wheels.