Buying rims for your car, nothing complicated you might say, it's 13″, 14″, 15″ or more with a width varying from 5″ to 10″ and there you go, the trick is done.
If only it were that simple, but what about the PCD (bolt pattern), the offset (ET), or even the brake calipers that no longer fit in the new rims you just bought because they have a slightly different shape than your old ones?
So how do we determine the correct dimensions?
Here is a short guide which I hope will help you determine the correct size if you ever decide to order new rims for your beauty or to have custom rims made as we offer with our rim manufacturer, the English company Team Dynamics.
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Let's start with the simplest thing, your center-to-center distance or PCD (Pitch Circle Diameter):
3 holes, 4 holes or 5 holes depending on your car and the measurement is relatively simple to take. 
Let's take the case of an Escort MK1 or MK2 for example. The PCD or bolt pattern is 4 x 108mm.
4 because 4 dowels
108mm = center-to-center distance of opposite studs: 
Let's now move on to the dimensions of your rim, using the diagram below for reference: 
Step 1: To take accurate measurements, it's best to do this with the wheel removed. Measure the ground clearance of your car, jack up the car, remove the wheel, and then lower the car to its desired ride height using leveling blocks to maintain the suspension at the correct height.
Step 2: Measure the Backspace / Dimension C
Place a ruler/or level vertically (from top to bottom) on the face of the wheel hub / photo Dimension C-1. Then, with a ruler, measure the distance between the inner face of your level (photo Dimension C-2) and the first point/part that could come into contact with the inner edge of the rim, this is usually a shock absorber/spring or suspension arm (photo Dimension C-3).
This measurement is your inside size C maximum possible (photo Dimension C).
Step 3: Measure the Frontspace / Dimension A
Place a level in the center of your wing, vertically and at a 90° angle to the ground (ensure the level touches the edge of your wing). Using a ruler, measure the distance between the level and the rim's mounting surface (see photos Dimensions A-1 and A-2).
Measure the thickness of your wing edge/return (photo Dimension A-3) and subtract this dimension from the measurement found (dimensions A-1/A-2); this will give you your outside size. HAS maximum possible.
Step 4: Measure the X Factor
Place a ruler on your caliper, parallel to the bearing face of the hub, and measure the distance.
The XFactor is an indication, a measurement taken at the center of the rim arm.
Step 5: Measure the Center Bore (centering hole) 
So that covers taking measurements, but what about the famous ET or Offset (offset)? This offset is usually indicated on one of the rim spokes, but sometimes without the ET marking, as follows:
A 6x15 rim with an ET 45 (positive) offset
A 10x15 rim with an ET-46 (negative) offset
But what exactly does that mean?
Simply put, an ET0 means that the distances A (front space) and C (backspace) are identical, therefore the wheel offset = 0
A positive ET indicates that the rim moves inwards relative to its absolute center (Distance A less than distance C).
And a negative ET indicates that the rim protrudes further outwards from its absolute center.
Let's take, for example, a Revolution 8 Spoke Classic Rally rim for a Porsche 911 in 8×15 ET11: 
This rim therefore has a PCD/Bolt Circle Diameter of 5/130 (5 studs and 130mm spacing) and the following dimensions:
For example, here are 2 Revolution 8 Spoke Classic Rally rims for Ford Escort in 8×13, the one on the left with an ET-6 and the one on the right with an ET-12 (and therefore with a larger outer rim of 6mm): 
Hopefully this short guide will answer your questions, but especially if you have any doubts when ordering rims, don't hesitate to Contact us.
