I asked this question in the middle of another post ,but no one had an anwser. So, I'm giving it it's own post.
I just notcied that alot of rear wheel drive sports cars have a bigger PROFILE on the front tires. Why is this? Should this be done on the Z? Like 225/45/18F and 255/40/18R?

 

This is done to keep the rolling diameter of the tires the same front to rear. Our cars have a computer that is set up for the stagger of diameter, so our traction control, ABS work with different diameter tires without problems .

 

45% of 225 = 101.25
40% of 255 = 102

 

Not too sure what that all means?

 

The profile, or "series" of the tire dictates how tall it will be proportionaly to the width. Having a 225/45 series tire will be shorter or "thinner" than a 225/55 series tire. And, having a 275/45 tire will be taller than a 245/45 tire.

The stock setup is staggered with the fronts being a slightly lower profile (225/45 front vs. 245/45 rear). This means the overall diameter of the front wheel/tire combo is smaller, and the VDC is setup this way....one knoledgable guy i talked to said that changing the diameter of the front wheels wont affect the ABS though.

So if you change the series of tire up front, youll have larger rolling stock up front, which will afffect how the VDC/TCS function, and will visually change the stance of the car, as it will raise the front end up a tad.

what julian was saying, is that a 245/45 tire and a 255/40 tire are gonna havethe same overall diameter, so they are interchangable.

 

The stock setup is staggered with the fronts being a slightly lower profile (225/45 front vs. 245/45 rear).
How are the front lower profile when the front and back have the same 45 profile?

 

The profile number (45 in this case) is propotional to the width. So the stock has wider 245 tires in the rear, and skinnier 225 tires up front, so since both are 45 series tires, the fronts are gonna be slightly "shorter" Just multiply the width by the series as a percent (245 X .45)

 

If you have a bigger width you'll have a wider tire that I know.But dose that also mean that if you have a bigger width with the same profile number you'll have a longer tire from top to bottom? A 245/45 tire is longer then a 225/45? Is this right?

 

So I just put a set of S03's with 245/40 front and 255/40 rear, what could this do to the VDC?

 

The "225" or "245"is the tire section width in mm. The "45" or "40"is the aspect ratio, which is the section height in mm divided by the section width in mm x100. Its a ratio and not a measurement. Aspect=(h/w)100. If you slide the decimal two places left and use .45, you can then multiply by the section width to find out the height h=.45x225 or h=.40x245

So, if you have the same aspect ratio but a tire with a wider section width (say 275), you have a tire with a greater section height h=A*w/100. The bigger or fatter the section height, the bigger the tire diameter will be (assuming they are tires for the same size rim). The aspect ratio has to go down as the width increases to keep the same height and visa versa.