The Wheel Offset of your tyres and wheels is the distance between the top of the hub mounting surface and the centerline of the tyre of a wheel. Illustrating right from the center of the width of a tyre, the center of a tyre whose width is 250mm’s will be 125mm’s apart from either side of the line.
The hub mounting surface is the area of the wheel that is physically tightened to the axle and is usually attached to the brake hub or rotor.
Outsets, also referred to as negative wheel offsets, are instances where the interior to the line of center of the tyre cleaves to the top surface of the hub mount. Negative wheel offsets, or outsets, are set in such a way that the wheels sit further apart on the axle thus like to make wheels protrude from the wheel arch of the car body.
Positive offset, as shown in the illustration below, indicates that the more the distance between the center line and the top of the hub mount, the deeper the tyre sits on the axel. Considered along with the body of the car, this implies that the wheels with higher positive offsets sit deeper under the wheel arch. The positive wheel causes the wheels to sit closer together and are called insets.
Closely associated with offset is the concept of backspacing. Backspacing and offset aren’t the same thing, though. Backspacing refers to the distance between the inner flange of the wheel and the mounting plate.
Hence, it is the combination of the wheel width and the offset. What this means is that the measure of backspacing is proportionate to the offset - an increased backspacing leads to higher positive offset while decreasing backspacing causes higher negative offset.
If at any point you get the backspacing or offset wrong, you’re at the risk of having the wheel sit too distant inside the wheel well. This can cause the tyre and the inner edge of the wheel to rub against the suspension. Wheel rub is a pressing issue already; it degrades the suspension components and the inner flange of the wheel. However, tyre rub can as much rip up the inside sidewall of the tyre. This area of the tyre is difficult to see if damaged and may remain undetected until the tyre blows out.
That explains why you don’t want to adjust your wheel offset more than 5 mm’s from its recommended offset fit. Five mm’s is the maximum leverage, so you have either way if you want to try experimenting with your wheel offset.
The majority of cars have their offset data written in the ‘Tyre Guide’ for the car’s specific year. Some websites also help in this section, such as WheelSizeCalculator.com. This website provides an array of useful data for any make and model, including proper wheel width, offset, hub diameter, and correct bolt pattern for each possible tyre and wheel size.
Most wheels have their offset measurement embossed on the back of the wheel, likely on the rear of one of the spokes. Some have it on the mounting plate. As an abbreviation for a German word of the same meaning, Offset measurements usually end with the letters’ ET.’ Accordingly, you will see a 45 mm offset lettered as either 45ET or ET45.
Wheel Offset Calculators are available to you find the wheel offset and backspace measurements, so you can correctly position the tyre/wheel assembly on your vehicle. These Calculators are a convenient approach to make an informed choice on which wheels are the fittest for your car.