I've got some fairly new Potenza's(18") on the front right now...225/45's.
I want to get some new wheels and tires. I know I want 245/40's on the front, and 275/40's on the rear. Question is, would it cause any problems to put my 225/45's on the front for now with the 275/40's on the rear?
Money saved by utilizing my existing front 225's would give me that much extra toward wheels. What you guys think?

 

You shouldn't have a problem as long as the overall diameter of the wheel / tire is as close to stock size as possible. A lot of guys are going to warn you about mismatching tires, but as long as the tread's decent, I don't think there should be a problem, but hey, I'm no expert.

Use a tire size calculator to check, like this one.

 

If you're not pushing the limits of the car (and tires), I don't think there's too much to worry about...the problem arises in that different tires will behave a little differently when pushed to their limits and thus if your front and rear tires are different, that could make your car more unpredictable.

If the overall diamter of the wheel (rim and tire) ratio of front to back changes, that could cause the VDC /TCS to activate...but you could just turn those off.

tread width * aspect ratio = side wall hieght
235 * .5 = 117.5 mm
275 * 0.4 = 110 mm

So, your new rear wheel will rotate slightly faster when compared to the front. (small diameter at same speed will require faster rotation)...in all it's not much of a % difference, but I don't know the tolerances the ECU has for the VDC/TCS.