Wheel offset explained: choosing ET without guessing

ET is the offset in millimetres: the distance between the wheel centreline and its mounting face. Positive offset places the mounting face towards the visible outside face of the wheel. Zero offset puts it on the centreline. Negative offset places it towards the inner side. Think of the hub as staying still while the wheel moves around it.

For two wheels of identical width, lowering ET moves the whole wheel outwards. Raising ET moves it inwards. An ET35 wheel sits 10 mm further out than an ET45 wheel of the same width. Neither number is inherently correct or sporty. The suitable position depends on the car and the wheel dimensions.

Read the markings on the back of the existing wheel rather than assuming the fitted wheels are original. Record diameter, width and offset separately. An 18 × 8 ET45 wheel has an 18 inch bead-seat diameter, an 8 inch bead-seat width and 45 mm positive offset. The overall rim is wider than its nominal width because of its flanges.

Changing offset alone is easy to picture. Changing width as well is where many expensive mistakes start. Every extra inch of nominal width adds 25.4 mm overall. With the centreline unchanged, half of that, 12.7 mm, appears on each side. A lower offset can recover some inner space while making the outer change much larger.

Compare an 8 inch ET45 wheel with a 9 inch ET35 wheel. The extra width adds 12.7 mm inwards and outwards. Reducing offset by 10 mm then shifts both edges outwards. The new inner edge reaches 2.7 mm further towards the suspension, while the outer edge moves out by 22.7 mm. Saying the new wheel is only 10 mm further out misses the width change.

Use a known setup as your baseline. With the car on level ground at normal operating load, identify the narrowest inner and outer gaps. A tyre can be close to a spring platform halfway up its sidewall even when the rim clears easily. The outer limiting point may be an arch liner or bumper mounting rather than the painted top of the arch.

• Record the current wheel and tyre specification on each axle.
• Have the inner gap measured to the nearest relevant suspension component.
• Measure the tyre's outer shoulder relative to the arch, including both sides of the car.
• Compare the proposed width and ET, then check the exact tyre dimensions.
• Arrange a trial fit and clearance assessment through steering and suspension travel.

Suppose the existing inner gap is 12 mm and the proposed wheel uses another 2.7 mm. The simple estimate leaves 9.3 mm at that same point. That is useful for screening a shortlist, but it is not a universal safe margin. The tyre may be wider than the old one, and a moving component may become the limiting point elsewhere. Ask the fitter to define an appropriate margin for the actual combination.

A spacer reduces effective offset by its thickness. An ET45 wheel with a 10 mm spacer occupies the same nominal sideways position as an ET35 wheel of the same width. That does not make the assemblies equivalent in every respect: spoke shape, hub engagement and the fastening arrangement still differ.

If a calculator suggests a spacer to restore your previous inner wheel position, treat that as a geometric result. It has not chosen a manufactured spacer, checked hub lip length or specified wheel bolts. A spacer also spends outer clearance, so solving a spring-platform problem can create an arch problem. Compare a different wheel specification before treating spacers as the default answer.

• Check spacer sizing and installation requirements

Two wheels with the same diameter, width and ET can have different brake clearance. One may use a flatter spoke face; another may have a barrel profile that narrows near the caliper. Offset does not describe either surface. Obtain the brake template for your exact brakes and the wheel drawing for the exact part number.

Moving a wheel sideways also changes its relationship to the steering geometry and bearing loads. A car that clears its arches can still steer differently over bumps. Avoid assuming a dramatic ET change is harmless because a similar-looking car on a forum uses it. Suspension layout, tyre construction and the amount of change all matter.

Lowering and alignment add another layer. The wheel may lean in at the top while the tyre still moves into the liner on lock. Do not choose an offset around an extreme camber setting simply to hide excessive width. Choose the intended ride height and a suitable alignment first, then specify wheels that work through the full range of movement.

Make a comparison sheet with three candidates, not twenty screenshots. For each wheel record width, ET, tyre model, estimated inner change and estimated outer change. Add separate boxes for brake clearance, load rating and supplier confirmation. A candidate stays provisional until every box has evidence behind it.

Take that sheet to the fitter before paying for tyres. If two options give the look you want, prefer the one with a straightforward approved fit and room for normal use. Keep the original measurements afterwards so any future suspension change can be assessed against a reliable starting point.

• Compare your wheel width and offset