How far does a wheel ET (offset) difference move the wheel?
When buying new wheels, the value most often overlooked is ET, or offset. Even if the diameter and bolt pattern are correct, a wheel with the wrong ET will rub on the wheel arch, touch the shock absorber or upset the way the car drives. In this guide we show how to see the effect of an ET difference with a simple calculation.
A reminder about ET
ET is the distance in millimetres between the face where the wheel mounts to the car and the exact centre of the wheel. If the mounting face is outboard of the centre, the ET is positive. Almost all passenger cars use positive ET, usually between 15 and 55.
- If ET increases, the wheel moves inwards towards the car.
- If ET decreases, the wheel moves outwards.
If only the ET changes
If the wheel width stays the same and only the ET changes, the calculation is very simple: the wheel moves by the difference.
Example: Factory wheel 7Jx17 ET45, new wheel 7Jx17 ET38.
- ET difference: 45 − 38 = 7 mm
- The wheel moves 7 mm outwards on both the inner and the outer side.
If the width also changes
If the wheel width also changes, you need to calculate two separate values: how much closer the inner edge of the wheel gets to the shock absorber, and how much closer the outer edge gets to the wheel arch.
First convert the width to millimetres: 1 inch = 25.4 mm.
Inner edge shift = (new width − old width) ÷ 2 + (new ET − old ET)
Outer edge shift = (new width − old width) ÷ 2 − (new ET − old ET)
A positive result means that edge moves closer to its own side (towards the shock absorber for the inner edge, towards the wheel arch for the outer edge).
Example
Factory: 7Jx17 ET45. New: 8Jx18 ET40.
- Width difference: (8 − 7) × 25.4 = 25.4 mm, half of which is 12.7 mm
- ET difference: 40 − 45 = −5 mm
- Inner edge: 12.7 + (−5) = 7.7 mm closer to the shock absorber
- Outer edge: 12.7 − (−5) = 17.7 mm closer to the wheel arch
An outer edge sticking out by almost 18 mm is enough to rub on the arch lip on most cars. The problem becomes noticeable especially when the car is loaded and the suspension compresses in corners.
This calculation only shows the rim edges. The tyre sidewalls bulge slightly beyond the rim edge, and if the tyre width has also changed you need to allow extra clearance for that.
How much difference is acceptable?
There is no fixed limit, as every car has different arch and suspension clearance. Generally accepted recommendations:
- Staying within ±5 mm of the factory ET is trouble-free on most cars.
- An outer edge moving out by more than 10 mm needs careful checking.
- There should be at least a few millimetres of clearance between the inner edge and the shock absorber or spring seat.
What else changes when ET changes?
- Track width: As the wheels move outwards, the car's track width increases. It looks wider, but the leverage on the wheel bearings increases.
- Steering feel: The scrub radius changes. The steering may pull more under braking or when hitting a pothole.
- Bearing life: Moving far from the factory value can make the bearings wear out sooner.
Using spacers
To lower the ET, a spacer can be fitted between the wheel and the hub. A spacer reduces the ET by its thickness: ET45 wheel + 10 mm spacer = effectively ET35. If you use spacers, choose hub-centric models and wheel bolts of the right length, and avoid poor-quality products.
The wheel fitment tool on Cars-Size does this calculation for you: enter the factory and new wheel sizes and see the shift at the inner and outer edges in millimetres.