What would happen if I put a rim with a 48 offset on the tC?

you'd rub on your struts?

Moooooved to the Tread and Butter section. Title also edited.

It really depends on how wide the wheels are...

This is the info I got from the wheel/tire faq, but I'm not sure if they are correct...
http://www.scionlife.com/forums/viewtopic.php?t=348

Bolt Pattern
5x100

Center Bore
54.1mm

Nut Size
M12x1.5

Stock Wheels/Tires
17"x7.0" +45 offset / 215/45-17
?? lbs per set

We offer a wheel with the following specifications which is an approved fitment for the tC - 17X7, 5-100, ET48. When used with a 215/45R17 or 225/45R17 tire would be a trouble free/rub free fitment on a tC which has not been lowered. If you have any additional questions, please fee free to contact me or visit our web site at:
http://www.tirerack.com/a.jsp?a=DJ1&url=/wheels/index.jsp