If you are trying to figure out what rims will fit you car or truck, here's a little tip to help you out. Typical FORD prior to the early 1980's were five on four and a half, or five on five, or five on five and a half (5 X 4.5, 5 X 5, 5 X 5.5) and CHEVY were five on four and three quarters (5 X 4.75). Since then both were still using the same patterns, but also went to using metric on smaller vehicles. A simple way to measure your circle with the rim off, is to take a tape measure and start with a stud on the far side (or back side) then skip a stud and measure to the center of the stud across from it. If your car or truck is Ford or Chevy and is prior to 1980, it should be in the sizes above. If not, most likely it is metric. This only works on vehicles with 5,6, and 8 lugs. If your car has four lugs, it cannot be measured this way and will most likely be metric with patterns of four by one hundred (4 X 100) and slightly larger on up.For example, Honda's such as the Accord and Civic are 4 X 100 mm and 4 X 110 mm and are not interchangable with most other japanese car or trucks like Toyota , Mazda, or Nissan. One last note to remember before buying those nice looking rims, is to take into consideration the back spacing of the rim to make sure it wont rub against the fenderwell when you make a turn, and to make sure it will fit in the well itself with clearence. If you still think you might need help, most rim makers and distributors will have every fitment possible for every car or truck made and will always be willing to give you that information for free if you ask. 

As of July 07,07 I am adding the following to this guide:

It has come to my attention that there are a few cheap and simple tools that can be purchased at your local automotive or convience store that will help identify the bolt circle in a split second.One such tool looks like a micrometer but is just a simple slide type of gauge made of plastic that when inserted into the rim itself will tell you the exact bolt circle whether it's standard or metric.This tool can also be used on the axle itself in case you don't have the rim that goes on it.You can find this tool here on E-BAY for about $15.00 or so with out shipping under the vintage parts section.The other tool is a round or circular disk with about ten holes in it that only fits american autos and trucks.This tool is only used on the axle itself and measures standard bolt circles such as FORD,CHEVY,and MOPAR only.This tool runs about $5.00 to $10.00 on E-BAY with out shipping and is also made of plastic.The first tool listed is better since it will measure the circle no matter how may lugs are in the pattern.The second tool is limited to five lug vehicles only.No matter which tool you choose,go with the cheapest one that will measure multiple circles.

I hope this helps.Best of luck to all!