Can somebody explain to me what all the numbers mean when your taking about size? 225/45/18 -> is the 225 the height of the tire? The 45 is the width, and the 18 is the size(height )of the rim? Also are you talking about when you talk about the offset?

Help someone who wants to learn.

 

All I know is that the 225 means its the width of the tire that comes in contact with the ground, 225 millimetres, wider the more traction. The 45 means that 45% of the 225 width makes up the sidewall height, so the lower the number the lower the sidewall and more performance oriented, but more bumpy ride.
The last number 18 is the the size of the rim (in inches) that the tire will fit on.
I dont know much about the offset, but from what everyone else on the forum says, its important to get a proper offset when buying new rims so that the tire doesnt rub against the car's fenders/bumpers. can someone else explain the "offset" concept?

 

I think this is in the FAQ, but if not...
Those numbers are a mix of meteric and US. Like Mojo said, the first number (225) is the width (more accurately = tire patch) in milimeters. 45 means the sidewall is 45% of the tire patch (101mm in this case). 18 means the diameter of the wheel it fits. Now, two tires out there may be the exact same size, but the sidewall can be a lot different. Some companies have rounder sidewalls, others more square; some soft, some firm. A 40 series tire (eg. 225/40/18) may have the same firmness of sidewall as another companies 225/45/18. It is important to match the tire to the correct size wheel. If your tire is not wide enough for the wheel, the sidewall will slanting in, and that can lead to broken beads, improper wear, and just looks WRONG. Too wide of a tire and the sidewall bulges out. The tire may roll off the wheel under hard cornering. Not good. Also, when plus sizing (increasing your tire/wheels from factory specs), make sure the overal diamter is ~the same. Another thing is that if you do not lower your car, but you get a plus size tire of the exact same height, there will be an optical illusion making the wheel gap look bigger than it is.
Offset. This one is harder. It is measured in mm, also, and is the distance the wheel is from the center line. I think the TLS stock wheels are 50? A plus offset will move the wheel towards the center of the car (tucked in), and a more negative offset wheel will be further out from the car. So, if you get a wheel with a less offset than stock, it may be flush with the fender, but if the offset is a lot more negative (40), the wheel may stick out further than the fender.
You need to think of how you want your ca in the end. If you add springs, offset matters. You may have to roll the fenders, or cut the plastic tabs inside the wheel well so there is no rubbing; usually needed in the rear with a drop of more than 1.5", or even more subtle drops if the offset isn't enough (eg. 42). Some people say that it matters which brand of springs, but I doubt that. The springs are the same size (diameter of the coils), the wheel still travels in the same plane, so if they both drop the car the same amount, how can it matter? It might be different with coilovers, though.
Anyway, this has gone on too far. I hope I didn't confuse you more (I think I am confused). Any more questions, let us know. For ideas, go to the tire/wheel forum and check out the sticky at the top. People say what size they have, and if there are any problems.