It's important to use recommended wheel widths and tyre size combinations - the wheel width dictates what tyre sizes you can mount. When choosing the wheel size, you should also remember to take into account the width of the car tyre at the point of contact with the wheel.

The wheel width is usually slightly smaller than the tyre width in order to ensure the tyre fits well. For that reason, the 185mm wide tyre is usually fitted to a 6-inch wide car wheel, even though 185mm is much closer to 7 inches.

What is wheel offset?

Wheel offset is the measured distance from the wheel's hub mounting surface to the centre line of the wheel. It lets you know (in inches or millimeters) how much the wheel will stick out from the hub mounting surface. The wheel offset can be positive, negative or zero depending on how the tyre sits in the wheel.

Positive offset means that the mounting surface is closer to the face of the wheel while negative offset describes the mounting surface situated towards the back of the wheel. Zero offset refers to the mounting surface being lined up with the centre line of the wheel.

It's crucial to apply the correct wheel offset because too much positive or negative offset can cause damage to the vehicle, especially when driving at greater speed. It might even cause a tyre burst in the end.

How can I tell what offset my wheels are?

Offset is stamped on a wheel as an ET value, which refers to the German phrase “Einpress Tiefe.” ET indicates the distance between the mounting surface and the geometric centre of the wheel (the symmetry axle), usually expressed in millimetres.

When the ET value decreases, the alloy wheels will protrude further out. An increased ET value, on the other hand, results in moving the wheel location deeper into the wheel arch.

However, the offset is not the only factor responsible for the location of the wheel against the wheel arches. For example, although it still has the same offset of ET30, a 9-inch wide wheel will protrude more than a 6-inch wheel. Why? Look at the following diagram:

The width and the wheel ET values - myths and facts - wheel offset guide

There are a number of myths surrounding wheel installation which are often mistakenly assumed as facts by unaware drivers. Here we will try to verify the key points addressing most common questions we hear when replacing car wheels.

"Is it possible to easily change the wheel offset within the range of  + / - 15?"

We mustn't generalise here - each case of wheel offset change should be considered on an individual basis. Car manufacturers actually allow a change of the wheelbase to a certain degree, which is usually around 2%. For example, when using a wheelbase of 160 cm, an acceptable change will be 3.2 cm. This results in a maximum allowed offset change of 16 mm per side provided that the width of the wheel and tyre size will not be changed.

Individual cases should also consider how much space the car manufacturer has assumed between the wheel arches. Any offset change, or change to the wheel width, could cause the wheels to rub against this arch, especially when under a heavy load.

Increasing the offset then, may also cause the risk of the wheel rubbing with the car brake calipers. Therefore, when planning any ET changes, the standard distance between the wheel and any crucial points of the vehicle should be carefully investigated. This includes the edges of the wheel arches, brake calipers, MacPherson struts and other nearby components.

While aftermarket wheels are made in multiple offset modifications, most OEM wheels come in the offset appropriate for a given vehicle model. Most wheels in today's cars are installed with positive offset.

"Is lower ET acceptable when it comes to alloy wheels?"

It does not matter whether you have alloy or steel wheels. The wheel must be located in the very same place, so the material does not often come into consideration.

However, when changing from steel to alloy models, it is often noted that steel wheels are narrower than alloy ones. This usually implies making fitting adjustments for wider wheels, which also means modifying other values.

If you can’t use a wheel offset calculator, there is a very simple formula to calculate how far the new wheel will protrude:

"Can I fit 7.5Jx16 ET35 wheels to a Renault Laguna?"

Renault recommends standard wheels with a 15-inch diameter, 6-inch width and ET45. Here the question is whether wheels with a 16-inch diameter, 7.5-inch width and ET35 can be used instead.

If we use these values in the formula presented above, the causes an increase in width by 28.75mm. The front wheelbase is 1480 mm, which means an extension of 14.8 mm is acceptable on either side. From these calculations, these wheels cannot be recommended for this particular vehicle.

"I would like to use wider wheels, what ET should I use?"

You want to know how to measure wheel offset? An appropriate offset value can be easily calculated to keep the wheelbase unchanged. To do this, simply use the formula presented below. Enter the parameters of the factory wheel and the width of the new wheel. The resulting value shows which ET is appropriate in this case.

The following table shows acceptable wheel and tyre widths according to the German TÜV organisation. When analysing them, consider any potential changes between your summer tyres and winter tyres as well. The guidelines of a German entity are not legally binding on English markets, but they are worth paying attention to.

In car tuning, it is common practice to introduce “German style” modifications, which refers to a ‘low and wide’ alteration. The installation of sports suspension systems, for instance, typically lowers the vehicle by around 40-120 mm, which usually requires narrow tyres with alloy wheels.