How to understand and measure car wheels.

So many people seem to get confused by all the numbers, so here’s my guide.

1. Most wheels will have the diameter and with cast into the inside of the spokes. Many newer wheels will also have the offset cast or stamped into them.
   Simply reading this information will save you some work!

2. Diameter is the measurement of the tyre that fits onto the wheel.
   If the wheel has tyres fitted, you can simply read the last figure of the tyre size.
   

If there’s no tyres and nothing cast into the wheel, then you can measure it either of two ways.
A) Measure it directly. Note that the measurement is taken from the inner area where the tyre sits, NOT the outside edge.

B ) Measure the circumference and then divide by 3.1416.
In this case, 47/3.1416 = 14.96, which is close enough to 15”.

3. Width.
   This is the easiest measurement to take!

In this case, the wheel is 7” wide.

4. Offset.
   Lots of people confuse themselves with wheel offsets. A wheel with zero offset will have the mounting face to the hub exactly in the middle of the edges of the rim (“middle” being side to side).

If you have positive offset, the mounting face is further outwards (blue line).
Negative offset is further inwards (green line).

More positive offset gives more “dish” like the street machine crowd loves.

Measuring it is easier than most people think, but it does take a little bit of brainpower.

A). Measure the back space.

In this case, its 110mm.

B ). Measure the “front space”.

In this case, the measurement is 92mm.

C). Work out the difference between the two measurements: 110 - 92 = 18mm.

D) Halve the difference: 18/2 = 9mm.

That’s the offset: +9mm.

This also works if the wheel has a tyre on it - they tyre sits symmetrically on the wheel, so you can lay your straight edge on the tyre sidewall and you’ll still get a useful measurement.

Be aware of tyres with “sidewall protectors” on one sidewall only - they can lead you astray, but you can usually estimate the difference accurately enough to get a close estimate of the offset.

5. Stud pattern.
   If you have 4-stud or 6-stud wheels, this is dead easy: just measure between opposite studs and you’ll get the PCD (pitch circle diameter).

Technically, you are measuring from the centre of one stud hole to the centre of the opposite stud hole, but this is a turd to do with simple tools.
Trying to guess the centre of each hole is difficult and likely to be inaccurate.

The much better way is to measure from the edge of a hole to the same edge of the other hole, like so:

If you have five stud wheels, then you can’t simply measure the PCD.

In this case, you have two options.

A) Measure between adjacent studs…
… and then use the following table to determine the PCD.

[table to be made. You can find this information on various trailer parts websites if you are impatient)

The maths behind this is the sine rule. I can explain it if anyone cares, but I am assuming that anyone who cares can figure it out for themselves!

B ). Measure the diameter of the stud hole (call this S).
Measure the diameter of the centre hole (call it C).
Measure the shortest distance between the edge of a stud hole and the edge of the centre hole (call this G).

Then add up: S + G + G + C = your PCD.

I don’t like this method much because you end up with a lot of tolerance stack up, but sometimes it is useful.

6. Communicating it.

The set of five stud wheels in the photos have the following dimensions:
15x7
5x11.4.3 PCD.
+9ET.
Along with the centre bore, that’s all of the commonly needed dimensions that should tell you (or a potential buyer) if the wheels will fit.
Some wheels may have issues with brake clearance, either from the inside of the spokes, or the inside of the barrel, but there’s no good way of measuring or communicating that information.

HTH.