This page covers the technical aspects of car wheels in terms of their physical dimensions and how to measure them. We’ll also take a look at the most commonly found markings on wheels and explain how to interpret them.
Diameters, Widths and Offsets
The diameter, width and offset of a wheel provide us with enough information to understand and visualise its external dimensions. This is important to know when choosing new tyres, comparing one set of wheels against another or if you’re attempting to understand how (and if) they will fit your car.
- Diameters and Widths
The most obvious and general descriptions of any set of wheels are their diameters and widths. On most modern production cars diameters range from around 14” up to 20” with widths in the region of 6” to 10”. Of course there are exceptions to this and it’s not uncommon for sports and modified cars to be fitted with staggered wheels (wider at the back than the front) for styling or performance reasons.
Both diameters and widths are measured from where the bead of the tyre sits on the rim. Diameter is generally obvious or already known, and it’s always measured in full inches. Standard wheel widths can be full inches or half inches and this is often measured incorrectly. Because a wheel’s lip protrudes by around ¼” from its measured width, it becomes easy to accidentally overstate a wheel’s width by ½”.
The red line shows where to measure:
One of the largest causes of confusion with regard to wheel terminology is when the letter “J” is wrongly used to describe the width of a wheel. Using “J” to describe the width of a wheel is incorrect as the letter has absolutely no meaning with regard to the width of a wheel. “J” actually describes the profile of the rim which seats the bead of the tyre and this is often stamped into the rim immediately after the width marking. The reason we see this so often is only because “J” is by far the most common rim profile.
“ET” is used by wheel manufacturers to state the offset of their wheels in millimetres, where a typical stamp on a wheel might look something like “ET42”, or “ET-33”. Assuming you already know the width, the offset is simply a number that allows you to calculate exactly where a wheel will sit in relation to the arch (or any other fixed point) of the car.
In the examples below we’ll call the flat part of the wheel that sits against the hub of the car the “rear face”.
– If this rear face was exactly in the middle of the wheel (e.g. 4” inside an 8” wide wheel) the offset would be “0”, or ET0.
– If the rear face was 5mm towards the front of the wheel, the offset would be +5mm, or ET5.
– If the rear face was the same distance towards the back of the wheel it would be -5mm, or ET-5.
Here you can see that a positive or negative offset just describes which side of the wheel the rear face is closer to. When comparing wheels in practical terms, a more positive offset wheel will “tuck” closer towards the car, whereas a more negative offset will result in the opposite effect.
While lower offset wheels do tend to have more “dish” – this is not always true as manufacturers can offer a lower offset wheel by increasing the thickness of the rear face, as opposed to extending the width of the outer lip. (Dish is simply the depth of the lip which protrudes from the face of a wheel, and in the case of most split rims it’s the depth of the outer lip itself).
The important thing to remember about offsets is that they are only meaningful if you also know the width of the wheel. Therefore there is no such thing as a “good” or “bad” offset unless you’re talking about a specific wheel for a specific car. Learn how to measure offsets in our wheel fitment guide.
Stud Patterns, PCDs and Centre Bores
- Stud Patterns
Most wheels have either 4 or 5 bolt holes which allow them to be secured to a car’s hub. These holes are always equally spaced, however the number of holes and how far away they are from each other varies from wheel to wheel. The correct way to describe a wheel stud pattern is by “PCD”, which stands for “Pitch Circle Diameter”. PCDs are stated as two separate numbers:
– The first number describes the number of holes in the wheel.
– The second number tells us how far apart these holes are in millimetres – if they were drilled through the circumference of an imaginary circle.
Some wheels come with “multi fit” PCDs, e.g. 4×100 and 4×108. This simply means there are two stud patterns per wheel to increase the number of vehicles that they may be fitted to.
- How to Calculate Wheel PCD
To measure the PCD of a wheel, imagine taking a compass and drawing a circle which passes through the centre of each hole. Since you already know the number of bolt holes, the diameter of the circle allows you to calculate the PCD of the wheel. In the example below, the black line is the PCD (100mm in this case).
- Centre Bores
The Centre Bore of a wheel is used to centrally located the wheel on the hub so that the wheel will not oscillate up and down whilst driving. If the wheel is not properly centred you may experience vibrations through the steering wheel – especially at high speeds.
The centre bore is hole in the very centre of a wheel which is measured by its diameter. If the centre bore isn’t already marked on your wheel (e.g. CB57.4), just take a measuring caliper (or ruler) and take note of the diameter. On a car with OEM wheels, the diameter of the protruding lip on the hub itself measures fractionally less than the centre bore of the wheel. This means that the wheel will be centrally aligned when the wheel bolts are tightened.
You should know the centre bore of your current wheels if you are buying a new set for your car. If the centre bores of the new wheels are smaller than the OEM wheels unfortunately there’s not a lot you can do to make them fit (besides machining). On the other hand, if the new wheels have centre bores larger than the old ones, these can be made to fit your car’s hubs with spigot rings as explained in our wheel fitment guide.
Wheel Nut/Bolt Types and Sizes
Wheel nuts and bolts come in either ball seat or cone seat (60 degree taper). Ball seat styles are common with OEM applications and cone seats are generally found with aftermarket wheels – but there is no hard rule here. (Note: Land Rover and other commercial vehicles may use bolts with a flat seat, but these are rarely used on road cars). It is crucial that the nut/bolt style matches that of the wheel itself, i.e. a ball seat bolt can only be used with a ball seat wheel. Besides the risk of damaging your wheels, the bolts will not be properly secured and may easily work loose.
- Nut and Bolt Sizes
A wheel bolt can be defined by 4 numbers: E.g. 17mm M14 x 1.25 55mm. The 17mm is the size of the bolt head and the 55mm is the length of the threaded section. M14 means the bolt is (metric) 14mm in diameter, and 1.25 is the thread pitch (i.e. the distance between threads in mm). You can check the thread pitch of a bolt with a gauge or compare bolts by holding them side by side – the threads will interlock if the pitch is the same.
Wheel nuts are described in the same manner, obviously excluding the threaded bolt length.
Tyre bead profiles – A continuation from “Diameters and Widths”
Without going into too much detail, the physical shape of a rim can be described by the initials: J, JJ, K, JK, B, P and D. For road wheels, 99% of the time the bead profile of the rim is “J”. A 15”x7” wheel with a “J” bead profile could be described as 15”x7j as is commonly seen in “for sale” adverts and on online forums. Occasionally you’ll see (often Japanese or off-road) wheels with a “JJ” bead profile and these wheels will still take regular tyres. The other bead types are extremely uncommon for modern on-road vehicles so we will not discuss these in further detail.
Although a “7j” wheel is always 7” wide, a 7” wide wheel does not necessarily use a J bead profile. If you are unsure, always describe wheels without stating the bead profile (e.g. 15”x7” instead of 15”x7j).