Anwan International : Wheels General Information - Sourcing & Outsourcing in China

Number of bolts or studs
It goes without saying that you can't fit a 4-bolt wheel onto a 5-bolt wheel hub.

Pitch Circle Diameter
You need to know the PCD, or Pitch Circle Diameter. This is the diameter of the invisible circle formed by scribing a circle that passes through the centre point of each mounting hole. If you've got the right number of holes, but they're the wrong spacing, the wheel just won't fit.

Offsets
This is very important. Ignore this and you can end up with all manner of nasty problems. This is the distance in mm between the centre line of the wheel rim, and the line through the fixing face. You can have inset, outset or neither. This determines how the suspension and self-centring steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can't turn the car, or so light that you need to spend all your time keeping the steering wheel in a straight line. More mundane problems through ignoring this measurement can range from wheels that foul parts of the bodywork or suspension, to high-speed judder in the steering because the suspension setup can't handle that particular type of wheel. This figure will be stamped on the wheel somewhere as an ET figure.

Wheel Sizes

Wheel sizes are expressed as WWWxDDD sizes. For example 7x14. A 7x14 wheel is has a rim width of 7 inches, and a rim diameter of 14 inches. The width is usually below the width of the tyre for a good match. So a 185mm tyre would usually be matched to a wheel which is 6 inches wide. (185mm is more like 7 inches, but that's across the entire tyre width, not the bead area where the tyre fits the rim.)

Rolling Radius

Rolling Radius is very important and need to be kept in consideration. This is the distance in mm from the centre of the wheel to the edge of the tread when it's unladen. If this changes because you have mismatched your new wheels and tyres, then your speedo will lose accuracy and the fuel consumption might go up. The latter reason is because the manufacturer built the engine/gearbox combo for a specific rolling radius.