The first thing i have to say regards alignment. AS SOON AS YOU CHANGE THE WHEELS ON YOUR CAR, GET A BALANCE AND 4 WHEEL ALIGNMENT!!!. This is one of the most important steps, especially for lowered vehicles, as lower profile tires won't be as forgiving on camber and toe and your tires will wear prematurely. The guys i knew on clublexus would run a few degrees neg. camber in the rear and have to flip their tires every 2 weeks and replace them every couple of months.
While we're in that area, i'll go into some definitions:
Camber: The angle of the wheel relative to dead 90 degree vertical axis down the center of the wheel. Ideally you want to be within .5 degrees of factory spec in either direction. Positive camber means the top of the wheel points outward, negative means the top leans into the fenderwell.
Backspacing: Not to be confused with offset, the backspacing is how much width you have from a vertical axis in line with the rear of the mounting hub, to the inner lip of the rim. This is especially important concerning those with BBK's or just large factory calipers.
Offset: This is a tougher one to understand, since it also affects other dimensions of the wheel. the offset of a wheel is determined by where the hub mounting surface is in relation to the centerline of the wheel. Positive offset means the hub is closer to the outer lip (most factory wheels have these offsets), usually referred to as a FWD offset. My old lexus had a +50 offset on the factory wheels). The more positive the offset, the larger portion of the wheel is backspacing. A zero offset means the mounting surface is dead on with the centerline of the wheel. Negative means the mounting surface is closer to the inner lip, usually the case with deep dish wheels.
Offsets are usually discussed in terms of 'aggressiveness'. This is a term that's often thrown around but seldom understood. It also depends on the car. For instance, a car like my old GS had a +50 offset on the stock wheels. A safe offset for aftermarket wheels on a car like this would be between 30-40. Aggressive comes between +18-25. One member fit a rear wheel on his GS (albeit with retarded negative camber) which was a 19x11.5 -51 offset. This means that the mounting surface is 5.1cm behind the centerline. now, since every 2cm is about an inch, this means the mounting surface was about 2.6 in. behind the centerline. A 0 offset would leave an 11.5" rim with 5.75" on each side. Subtract that 2.6in, and there is only 3.1" backspace, while the lip is a gigantic 8.3".
FWD cars, for sake of traction and the drive/suspension components on their front ends, should stay above 30 on their offsets. Negative camber adversely affects the front drive system. Same with all-wheel drive cars. There are exceptions, cars that have smaller brakes or more space between hub and suspension components.
RWD Cars can have the most fun with offsets. Obviously dependent upon your suspension setup and brakes, etc., you can get pretty crazy. My cobra comes stock with i believe a +28 offset wheel. Pretty aggressive by most standards, but this is why the front wheels are nearly flush with the fender. The higher the offset, the wider the wheel usually, and the more negative camber is needed as more of the wheel sticks out beyond the fender.
An illustration of what im talking about:
Now to another fun topic, spacers.
For this part, i'll be using the term "effective offset" a lot. This just means the remainder left over after the offset is added to the width of the spacer.
for instance, a +50 wheel is sucked way into the fender on the lexus. the going rate seemed to be a 20-30mm spacer. When you add the spacer, it's almost flushed. Thus, the 50 offset plus, say, a 25mm spacer (50-25) leaves an effective offset of +25.
Spacers over 10mm, to be used correctly and safely, SHOULD HAVE THEIR OWN STUDS. 3mm, 5mm, and even 10mm depending on the car, can be used as shims. Anything more should bolt itself to the hub, and have it's own studs to bolt the wheel to. These are a har more expensive (around $50 for 3mm or 5mm shim spacers, up to 275 for lug centric spacers), but your wheel wont shear a stud and fly off the car on the highway causing you to DIE.
Now, we need to talk about increasing diameter.
Increasing rim diameter has a similar effect to changing your rear end gear ratio, as many mustang and f-body guys know very well. To reduce strain on brakes and the drivetrain, it's recommended that your aftermarket wheel and tire combined diameter be within 3% of your stock overall wheel and tire diameter. This is so that it doesn't throw your speedometer off.
On a car that has a 15" wheel with a 55 a/r tire (4gen maximas mainly), switching to a 17" wheel with a 45 a/r tire will make your speedometer read 4mph slower than normal. A pain in the ass when you thought you were only going 5 over and you were actually going 9 or 10 and get pulled over.
For this, since the calculation is really complicated, i suggest using the tire size calculator at 1010tires.com. It's extremely accurate. You enter in your year, make, and model, and which tire sizes you're looking at getting, even staggered setups. It will tell you the percentage you'll be off by, and how much it will throw off your speedometer. For you mustang guys, corral.net also has a gear calculator that you can play with to see which gear set's speedo correction gear will straighten out your speedometer. Some computer controlled cars can be corrected, but they are few and far between. A good example of what we're talking about is throwing 22's on a chrysler 300 that came with 17's stock. Going to a 35/22 tire from a 50/17 is enough to throw your speedometer off up to 7 or 8 mph. Again, really bad when trying to watch your speed.
Wheels also come with a disk rating. I don't remember them exactly, i know one is r-disk, and maybe S and H? I know that they correspond to the following:
-high disk (will clear bbk)
-med disk (will clear bbk if big enough diameter)
-low disk (will not clear bbk)
-super low disk (MUST use stock brakes, maybe even another model's calipers if too big).
If anyone has anything to add, please do. If you have any questions, i'd love to help you as much as possible.
If you have a question about what can fit, post with what car it's going on, supsension mods, brake mods, and factory tire size.