I know its 245/45/18..
But can stock wheel use it 245/40WR18?

 

Yes it can, your speed will be thrown off about 2% slower though, not much to make a big difference.

 

what does that mean?

 

You'll throw off the stagger if you use those in the back. They are 1" lower than the 245/45s, therefore you will have 1/2" more space between the top of the tire and the fender well. If you have VDC, it wont like it. 245/40s are fine in front, because they are very similar to the stock 225/45s in overall diameter. Using the 245/40s in front and 245/45s or 255/45s in the rear will also lessen the oversteer.

Lou

 

so in turn the rear is unfeasible...
???

 

Yes

Lou

 

If my wheel require 245/40/18....and i use 245/45/18 on it.

what will happen?!