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

However, when choosing any wheel, the width of the car tyre, at the point of contact with the wheel, must always be taken into account.

The width of the rim is related to its offset. This is otherwise known as ET, 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), 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.
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 difference between a 6 and 9 inch wheel.

The truths and myths associated with the width and the wheel ET values

There are a number of false beliefs and assumptions regarding wheel changes, which can be mistakenly assumed as facts by unknowledgable drivers. Here, we will try to verify these key points, responding to some common questions that can arise when replacing car wheels.

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

We should be careful not to generalise here. Each case of offset change should be considered on an individual basis. Car manufacturers actually allow a change of the wheelbase to a certain degree, usually around 2%.
This means, using a wheelbase of 160 cm, that an acceptable change will be equal to 3.2 cm. This results in a maximum allowed offset change of 16 mm per side. Of course, it should also be assumed 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 accounted for 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.

"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 usually narrower than alloy ones. This usually results in the need of fitting adjustments for wider wheels, which also means adjusting other values.
Fortunately, there is a very simple formula to calculate how far the new wheel will protrude:


the formula for determining the new protrusion length of a car wheel.

"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. This question wants to know if 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 result is an increase of 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 recommend for this particular vehicle.

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

An appropriate offset can be easily calculated to keep the wheelbase unchanged. To do this, simply use the formula presented below. Please enter the parameters of the factory wheel and the width of the new wheel. The resulting value shows which ET is appropriate in this situation.


The formula to determine the ET value for wider wheel changes

Furthermore, the following table shows acceptable wheel and tyre widths, according to the German TÜV organisation. It should go without saying, of course, that you should consider any potential changes between your summer tyres and winter tyres as well.


Acceptable wheel and tyre widths, as according to TÜV.

Of course, the strict guidelines of a German organisation 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, requiring rather narrow tyres with alloy wheels.
If you are interested, a comprehensive gallery of unusual wheel and tyre combinations can be found at