Originally Posted by jsteg
Don't know about the spacers, maybe someone else can answer. I will however explain what I mean using your example. 225 is the tire's width in millimeters. 18 is obviously the the wheel or rim size. The 40 is the aspect ratio, which refers to sidewall height. However, it's not automatically 40 mm or anything like that. What the aspect ration means is that the 40 means the sidewall height is 40% of the width (225 mm). So in this case, the sidewall height is ~90 mm. Yes, obviously this does kind of go along with tire height, however, it's not quite that simple. For example, a 225/40/18 will be shorter in overall height than a 265/40/18, and the sidewall would be taller too. This is why a 205/50/17, a 225/45/17, and a 225/40/18 are all just about the same height tire overall.
I thought I already corrected this information in the previous post.
About the spacer. Well, I came frome a racing back ground and never modified my daily cars before that. What I learned is that if you just used a spacer without extended studs then when you out the wheels back on, there will be less threads holding the wheel. Its a big no-no in the racing would because of the high stress of wheel. But on a normal daily, I cant see this to be a big problem. Safe is relative. Safe for daily commute. Sure, low stress is demanded from the wheels. Safe for racing? Never, too high stress demand. I wouldnt risk it personally from experience. But again I cant see the danger of running a 5 mm without extended studs on the streets. 10 mm spacers would require longer studs for sure. I think u lose about 3-4 turns with a 5 mm spacer. But I will not be responsible for whatever happens. Always properly torque your lug nuts.