Wheel Offset Explained: ET, Positive vs Negative and Fitment

Hand-drawn wheel offset title card

Wheel offset is the distance, in millimetres, between the face where the wheel bolts to the hub and the centreline of the wheel. It decides how far the wheel and tyre sit in or out of the guard. Get it wrong and a set of wheels that bolt on perfectly can rub, wear bearings faster or push the tyre past the guard, which is illegal in Australia. This guide shows how to read the number, what a change in it does, and how to check it before you spend money on wheels or tyres.

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TL;DR

Offset (stamped as ET) is the distance in millimetres from the wheel’s mounting face to its centreline. A higher ET sits the wheel further inboard; a lower or negative ET pushes it outboard.
Most modern road cars use a positive offset. Negative offset is common on lifted 4x4s and deep-dish wheels.
A few millimetres either way is usually harmless. Changes of 10 mm or more alter track width, scrub radius, bearing load and guard clearance.
Australian rules cap how much wider a car’s track can get without certification, and tyres cannot stick out past the guard. Check VSB14 and your state’s rules first.
Offset only describes where the wheel sits. Width, tyre size and load rating still need to match, so confirm the full fitment before ordering.

What wheel offset actually is
How to read the ET number on your wheel
Positive vs negative offset: what changes
Offset and backspacing
What offset does to clearance and rubbing
Why a change in offset affects how the car drives
Australian rules on offset, track and guards
Wheel spacers and offset
How to check a new wheel before you buy
Where offset matters most by vehicle type
Our perspective
Get the tyres sorted once the wheels are right
FAQ
Sources

What wheel offset actually is

Picture a wheel cut in half through its width. The line down the middle is the centreline. The mounting face is the flat surface at the back of the wheel’s centre section that sits against the hub. Offset is the gap between those two, measured in millimetres. Wheel offset is stamped on the wheel as ET followed by a number. ET comes from the German word Einpresstiefe, which simply means insertion depth.

Three outcomes are possible:

Zero offset: the mounting face is exactly on the centreline.
Positive offset: the mounting face is outboard of the rim centreline, which tucks more of the wheel in towards the suspension.
Negative offset: the mounting face sits inboard of the centreline, so more of the wheel stands out from the hub.

Offset says nothing about how big the wheel is. A 7-inch and a 9-inch wheel can both be ET35, but the wider one extends further both inside and outside, because the centreline is further from each edge.

How to read the ET number on your wheel

Look on the back of a spoke, on the inside of the barrel or near the valve hole. You will see a string like 17x7.5J 5x114.3 ET40. Reading from the left:

17 is the rim diameter in inches.
7.5J is the rim width in inches.
5x114.3 is the bolt pattern: five studs on a 114.3 mm circle.
ET40 is the offset: 40 mm positive.

Negative offsets are written differently by different makers. Negative offset can appear with a hyphen, such as ET-12, or with an N suffix, such as ET12N. Read ET-12 and ET12N as the same thing.

If you cannot find the stamp, a wheel shop can measure it. Your car’s original offset is usually shown in the handbook or on the manufacturer’s fitment data, and the markings on your existing wheels are the fastest way to find what your car currently runs. Our guide to finding your car’s original tyre size covers the tyre side of that same check.

Positive vs negative offset: what changes

The simplest way to think about it is a dial that moves the wheel in or out relative to the hub.

	Positive offset (higher ET)	Negative offset (lower or minus ET)
Mounting face	Outboard of the centreline	Inboard of the centreline
Wheel position	Pulled inward toward the suspension	Pushed outward toward the guard
Typical use	Most front-wheel-drive cars, sedans, hatchbacks and many SUVs	Lifted 4x4s, utes, some rear-drive performance cars, deep-dish wheels
Main risk	Rubbing on struts, springs or brake calipers	Tyre sticking out of the guard, extra load on wheel bearings
Effect on track	Narrower	Wider

The middle ground matters most. If your factory wheel is ET45 and a new wheel is ET40, the wheel moves 5 mm outboard on each side. Most cars absorb that without any visible or measurable fuss. If the new wheel is ET20, you have moved the wheel 25 mm out per side, and everything in this article starts to matter.

A worked example

Say your car came with 17x7 wheels at ET45, and you are considering a 17x8 at ET35.

Outer edge: the extra inch of width splits across both sides, so each edge gains about 12.7 mm. The lower offset adds 10 mm outward. The outer lip ends up around 22.7 mm further out than before.
Inner edge: the extra half inch moves it 12.7 mm inward, and the 10 mm lower offset pulls it back out by 10 mm. Net, the inside moves only about 2.7 mm inward.

So a wider wheel with a lower offset pushes mostly outward. That is the combination to treat with care, because the guard is where you run out of room first.

Offset and backspacing

Backspacing is the other number you will see on wheel listings, especially American ones. It is the distance from the mounting face to the rear (inside) edge of the rim. In inches it works out as wheel width divided by two, plus offset, plus half an inch, with the offset converted from millimetres to inches.

For a 8 inch wide wheel at ET35:

Half the width: 4 in
Offset: 35 mm, which is 1.38 in
Allowance for the rim lip: 0.5 in
Backspacing: about 5.88 in, or roughly 149 mm

Both numbers describe the same position, so you only need one to compare wheels. Offset is the Australian standard, and it is the figure wheel stamps and fitment data use. Backspacing is useful when you are checking the physical gap to a spring or caliper with a tape measure, because it tells you how deep the inside of the wheel reaches.

What offset does to clearance and rubbing

Offset sets the clearance on both sides of the wheel at once. Too much positive offset and the inner edge of the wheel or tyre reaches the strut, spring or caliper. Too little and the outer edge reaches the guard. Increasing positive offset moves the wheel inboard, which reduces clearance to suspension and brake components. Going the other way, lowering the offset or running negative offset can push wheels and tyres too far outside the arches.

Positive and negative offset clearance comparison

A wheel can look fine on the stand and still rub on the road. Guard clearance at full lock and under suspension compression is critical, and a wheel that clears while stationary can still rub under load. That is why a good fitter checks three positions: straight ahead, full lock each way, and with the suspension compressed over a bump or with a load in the car.

Tyre choice makes this tighter or looser. A wider or taller tyre uses up the same room that offset does. If you are changing both, read our guides to how wheel and tyre sizes match and plus sizing tyres before you commit, and use the tyre size calculator to see how a new size changes overall diameter and speedometer reading.

If you are pushing the limits of clearance, it is worth having a workshop that knows your platform look over brakes and suspension. A shop that works on suspension and brake clearance on European and performance cars will often spot a tight spot before it becomes a damaged tyre.

Why a change in offset affects how the car drives

Offset does more than move the look of the wheel. Changing offset changes track width and scrub radius, which affect steering feel, behaviour under braking and the leverage on wheel bearings.

Track width: a more negative offset moves the wheels outboard, widening the track and potentially stabilising the vehicle. Wider track generally means less lateral weight transfer in corners, assuming everything else stays equal, as High Performance Academy notes.
Scrub radius: this is the distance on the road between the steering axis and the centre of the tyre’s contact patch. Pushing the wheel out changes it, which can make the steering heavier, more nervous under braking, or more prone to following ruts.
Bearing load: a wheel further out is a longer lever on the same bearing. The result is a wear problem over months and years, not a failure on the first drive, which is why it is easy to ignore.

The lesson is not that negative offset is bad. Plenty of vehicles are set up for it from the factory. The lesson is that every millimetre away from the original specification is a trade, and the bigger the change, the more the car’s engineers’ numbers stop applying.

Australian rules on offset, track and guards

If you are in Australia, offset is not just a fitment question. It is a compliance one.

Track limits: for passenger vehicles, track must not increase by more than 25 mm beyond the manufacturer’s maximum specified track, while off-road and commercial vehicles with beam axles at both ends may increase it by up to 50 mm in total. Beyond that, the vehicle needs certification. Check VSB14 and your state’s requirements for the current detail.
Guards: it is illegal for tyres or wheels to protrude past the wheel arch. A lower offset or a wider wheel are the usual ways to end up over the line.

Track is counted across the axle, so a 15 mm drop in offset per side adds 30 mm to the track. Go back to the earlier example: a switch from ET45 to ET30 on a standard passenger car would exceed the 25 mm limit before certification enters the picture. The limit is measured against the manufacturer’s maximum specified track, so a car that left the factory with several wheel options has a different baseline from one with a single option. Your state road authority or an engineer can confirm which applies to your vehicle.

The tyre also has to be legal for its job. Load rating and speed rating need to match or exceed the original, and our guides to tyre load index and tyre speed rating explain what to check. For the overall diameter limit, see how to calculate tyre size to NCOP11 limits.

Wheel spacers and offset

A wheel spacer is a plate that bolts between the hub and the wheel, moving the wheel outward by its thickness. A 15 mm spacer on a wheel behaves like dropping its offset by 15 mm: the mounting face is effectively moved inboard relative to the hub, and the wheel sits further out.

In Australia this is a legal minefield. Current guidance reported by CarsGuide is that state and federal rules, including VSB14, prohibit spacers between the mounting face and the wheel unless the vehicle manufacturer supplied them. If you need a wider stance, the cleaner route is a wheel with the right offset from the start, within the track limits above. Extra studs, thicker plates and longer wheel nuts also bring their own problems, so a spacer is rarely the shortcut it seems.

How to check a new wheel before you buy

Work through these steps in order:

Find your current wheel’s full stamp: diameter, width, PCD, offset. Photograph it.
Look up the factory specification for your model and variant, including the maximum track.
Work out the change in offset and width, and calculate how far the outer and inner edges move, as in the worked example above.
Check the legal limits: track increase and no protrusion past the guard.
Check the tyre: a size that suits the wheel width, with the right load and speed rating. Pirelli advises using the wheel and tyre equivalence table to find the minimum and maximum tyre widths a rim can take.
Test fit before mounting: put the bare wheel on the hub, turn to full lock, and check the gap to the guard liner, strut and caliper.
Have the fitter check at full lock and under compression once the tyres are on.

Do not skip centre bore and stud pattern either. They are separate from offset but the wheel will not seat without the right ones.

Where offset matters most by vehicle type

Everyday cars and SUVs. Stay within a few millimetres of factory offset and the tyre size will mostly take care of itself. Moving to larger rims is where offset and tyre sidewall height interact, so see our plus sizing checks.

Lifted 4x4s and utes. Bigger mud-terrain or all-terrain tyres need clearance, and wheels with lower offset are the usual answer. This is also where track limits and guard coverage cause trouble. Our guide to 4x4 tyre sizes in Australia covers how to read and check a size, and you can browse the full tyre sizes directory to see what is available in your fitment.

AWD vehicles. Tyres across the axles must stay closely matched in diameter, so a change in wheel and tyre package should be done as a full set. Reading on tyre matching and inspection for AWD cars explains why a mismatched tyre is a drivetrain cost, not just a tyre cost.

Where offset matters most by vehicle type — overview diagram

Our perspective

Most fitment problems we see start in the same place: someone buys wheels on width and looks alone, and only afterwards checks the offset. The wheel is fine. The combination is the problem.

Our view is that offset should be the first number you check, not the last. It decides how much room you have to choose a tyre, so settling it first makes the tyre decision simpler. Once you know where the wheel will sit, the tyre width, profile and load rating follow, and the whole package can be checked against the legal limits before anything is bought.

We also think the honest answer for most drivers is to stay close to the factory number. A few millimetres of change is fine. A big change is a project that needs measuring, a legal check and a fitter who will test at full lock. If the numbers do not work, change the tyre size or the wheel, not the rules of physics.

— Areesh

Get the tyres sorted once the wheels are right

When your wheel size is confirmed, we can help with the tyre side. Search by car model or tyre size, see the price clearly, choose a local fitter and book in a few minutes. Browse the range on our tyres page, or if you are in Perth or Melbourne, see tyre fitting in Perth or tyre fitting in Melbourne. If you are not sure what fits your car, start with our guide to finding tyres that fit your car.

FAQ

What does ET mean on a wheel?

ET is the offset stamped on the wheel, in millimetres. It is the distance between the mounting face and the wheel’s centreline, so ET40 means a 40 mm positive offset. A negative number, or a number with an N, means the mounting face is behind the centreline.

Is positive or negative offset better?

Neither is better by itself; the right one is the one your car was designed for, or close to it. Positive offset tucks the wheel in and suits most modern passenger cars. Negative offset pushes the wheel out, which suits some lifted 4x4s and wider stances, but it adds load to the bearings and risks breaching guard and track limits.

Can I run wheels with a different offset from the original?

Often yes, within a small margin, as long as the wheel clears the suspension, brakes and guard at full lock and under compression. In Australia the track increase and guard coverage are also limited, so check VSB14 and your state’s rules before you go far from the factory number.

How do I convert offset to backspacing?

For a wheel in inches, add half the wheel width to the offset converted to inches, then add half an inch. A 8 inch wheel at ET35 gives about 5.88 inches of backspacing. It describes the same position as offset, just measured from the inner rim edge.