Wheel Sizes - Simpler Than They Seem

While wheels are, in large, used for aesthetic purposes, understanding the sizing is critical to ensure safety and security on the road.

Below is a wheel diagram to help us visualize and explain the different components of a wheel.

Wheel Diagram

Let’s look at an example to help define the different aspects of a wheel size.

 18x8 5x112 ET40

Wheel Diameter (18):

As shown on the diagram, wheel diameter is the distance measured across the wheel face from bead seat to bead seat, in inches. It is calculated from the bead seats because this is where the tire sits on the wheel. The wheel diameter is the first number seen in the size. In this scenario, the wheel diameter is eighteen inches. 

Wheel Width (8):

 Like wheel diameter, wheel width is also measured in inches, from bead seat to beat seat, but across the barrel of the wheel rather than the wheel face. In this scenario, the wheel width is seven inches.

Wheel Bolt Pattern (5X112):

The Wheel Bolt Pattern is comprised of two numbers. The first number is the lug count or in other words, the number of bolt holes the wheel has. In this example, this wheel has five lugs/bolt holes. The second measurement is the distance, in millimetres, between two holes parallel from one another. As shown in the diagram below, the most popular lug counts are four, five, six, and eight. Since the second count has an odd number of holes, they are not directly across from one another, and their distance is measured slightly differently than the rest. Here, the distance is 112 millimetres. The diagram below provides a brief explanation of how to measure a wheel’s bolt pattern.

Wheel Offset (ET40):

Wheel offset is the distance, in millimetres, from the wheel’s mounting surface to the wheel’s centreline. Essentially, how far in or out the wheel mounts to the vehicle. In this scenario, the offset is ET40 or +40 millimetres, meaning the mounting surface is 40 millimetres away from the centreline. Wheel offset can be positive, negative, or zero. Wheel offset prevents two main issues from occurring:

1. The correct wheel offset ensures that your wheels and tires do not rub against the suspension, brakes, and fenders.
2. Wheel offset is also used for aesthetic purposing, determining whether the wheels sit tucked into the fenders, or pushed out.

• A positive wheel offset leads to a tucked-in wheel. For example, if you want to lower your vehicle’s suspension, you need to tuck the wheels in and therefore, have a positive offset.
• A negative wheel offset pushes the wheel past the fenders and provides a wider stance for your vehicle.

 Wheel Backspacing:

Backspacing measures the distance, in inches, from the mounting surface to the back of the wheel. Backspacing is an important measure as it provides an understanding of the clearance between the wheel and the brake callipers and suspension, ensuring the vehicle is equipped in a safe and performance-driven manner.

Backspacing and offset go hand in hand. A positive offset provides more backspacing, while a negative offset provides less backspacing and therefore, less clearance between the wheel and the vehicle's wheel hub. Check out the diagram below for a visual representation of a wheel’s offset and backspacing.