why dont they offer the same sidewall size for the rear tires, for example the wheel will be 19 inches but the tire for the rear would be 275/40/19 and the front tire will be 245/45/19 why dont they match the wall size to 45 for the rear also?
Because the second number is not a set dimension. The second number is called the aspect ratio, and it is based on the width. Taking the 275 width tire as an example, the tire is 275mm wide or 10.83"; the 40 means that the sidewall height is 40% as tall as the width, which is 4.33 inches. The reason that they use different aspect ratios is that the widths are different, the diameter of the 275/40/19 is 27.66" while the 245/45/19 is 27.68". They end up being almost exactly the same diameter. If you want to see what the different tire sizes will be, check out the tire size calculator here. Tire Size Calculator - Discount Tire Direct
I hate that they do that. Bought a set of 245/35 285/30 and it looked like crap. They're supposed to be the same diameter but you can noticeably tell the front tires we're taller thus ruining the stance. Stick with the same size.