When the wheel is 8 mm too far out, it is immediately noticeable – in the fender edge, in the steering feel and sometimes in the inspection. That is why it is not enough that “it should fit”. If you are going to change the wheel setup, increase the width or make room for larger brakes, you need to be able to measure and calculate ET rims for real, not by feel.
ET is one of those measurements that is often mentioned in passing, but which in practice controls how the car behaves and how much margin you have against the struts, brake calipers and fenders. For a street car, the wrong ET can cause unnecessary wear and poor fit. For track driving, drifting or rallying, it can also affect steering response, scrub radius and how the car works under load. In short – the right rim is not just about looks.
What ET on rims actually means
ET stands for offset. It is the distance between the centerline of the rim and the rim's contact surface with the hub, given in millimeters. If the contact surface is outside the centerline, you have a positive ET, which is most common on modern passenger cars. If it is inside the centerline, the ET is negative, which is more often seen on older builds, some off-road vehicles, or extremely wide setups.
This measurement determines where the rim ends up in the wheel well. Higher ET moves the wheel further in towards the car. Lower ET moves the wheel further out towards the fender. It sounds simple, but if you combine ET with a different rim width, everything changes at once. That's where a lot of things go wrong.
Measure and calculate ET rims – that's how you do it
There are two situations here. Either you have a rim without clear markings and want to find out the ET value. Or you compare a new rim against your current setup to see how much further in or out it ends up. Both are important, but they require slightly different thinking.
How to measure ET on a loose rim
Start by measuring the overall width of the rim from outer edge to outer edge. Many people make the mistake of using the nominal width, such as 8 inches, but that number usually refers to the width between the tire seats – not the overall outer width. To calculate the exact ET on a physical rim, you need to measure the actual overall width.
Then divide the total width in half to get the centerline. Then measure the distance from the hub bearing surface to the inner edge of the rim. The difference between that measurement and the centerline gives the ET value.
The formula in practice is simple:
ET = distance from plant surface to centerline
If the centerline is 63,5 mm from the rim and the contact surface is 48 mm from the inner edge, you get an ET of 15,5 mm, depending on how you measured the overall width of the rim. That's why precision is important. A measurement error of a few millimeters can be the difference between free play and contact at the fender edge.
Calculate ET when changing rim width
This is usually the most important thing for the person who is rebuilding the car. Say you currently drive 8×18 ET45 and are considering 9×18 ET35. Then it is not enough to just look at the ET number. You have to calculate how much both the inner and outer edges move.
One inch is 25,4 mm, so an increase from 8 to 9 inches means 25,4 mm more width overall. That increase is distributed by 12,7 mm in each direction if the centerline is kept constant. However, because you are also going from ET45 to ET35, the entire rim is moved 10 mm outward.
This means in this example that the outer edge ends up 22,7 mm further out than before, while the inner edge ends up only 2,7 mm further in towards the strut. This is exactly the kind of calculation you need to do before buying.
Therefore, ET plays a bigger role than many people think
On a standard car with plenty of room, you can get away with quite a bit. On a lowered car, a car with coilovers, larger brake packages or wider tires, there is significantly less room. Then ET affects both fit and function.
Too low an ET can give a nicer flush look, but increases the risk of the tire catching the fender during compression or full steering. Too high an ET can instead create problems with the struts, inner fender or brake calipers. If you install spacers, you also effectively change the ET value, which is good to use if you know what you're doing - and expensive if you take a chance.
There is also a driving characteristics side. When the wheel position changes, the load paths are affected through the hub, wheel bearings and steering. Small changes can work fine. Large changes can cause more kickback, heavier steering and faster wear. On the track, it can be noticed in how the car pedals into corners and how it behaves over curbs. On the street, it is often only noticeable when something starts to seize or feel “off”.
Common mistakes when measure and calculate ET rims
The most common mistake is comparing two ET values without taking rim width into account. An ET35 on an 8-inch rim and an ET35 on a 10-inch rim do not place the rim edges in the same place. This goes without saying when you think about it, but is still often missed.
The next classic is to confuse nominal rim width with measured outer width. If you are going to calculate on an existing rim with a folding rule, you need to know what you are actually measuring. Otherwise you will get the wrong centerline and thus the wrong ET.
Many people also forget about the tire. The rim may be clear with a 3mm margin against the strut, but the tire sidewall builds more than the rim edge and eats up that margin right away. This is especially true for tires with angular shoulders or wide actual section compared to the marking.
Finally – the brakes. A rim can have the correct ET but still not be able to handle the brake caliper due to the spoke design and the inside of the rim. So ET doesn't solve everything. You have to look at the big picture.
An easy way to compare old and new
If you already have a setup that works, that's your best reference. Start from that and calculate the change in two steps: difference in width and difference in ET. Then you can immediately see how much the rim moves inward and outward.
Take the width difference in millimeters and divide it by two. Then add or subtract the ET difference depending on whether the new rim has a lower or higher ET. The result shows how the outer edge and inner edge change. It sounds technical, but after two examples it makes sense.
This is especially valuable if you're building for function and not just for parking. If the car is going to withstand track attacks or hard road driving, you want to have margins even when everything is moving – suspension, roll, tire flex and heat.
When spacers are the right solution
Spacers are often used to adjust the wheel when the ET is too high. A 10mm spacer effectively lowers the ET by 10mm. A rim with ET45 will therefore effectively become ET35 with a 10mm spacer.
It can be a smart way to fit brakes or improve track width, but only if the rest is right. Hub alignment, bolt length and quality of parts are absolutely crucial. On a performance car under hard driving, there is no reason to skimp here. Choose parts that can handle the job and always check that you still have correct alignment and sufficient thread engagement length.
So you think right from the start
If the goal is a car that both looks right and performs under load, don't start by chasing the lowest possible ET. Start with the car. How much room is there inboard? How much room is there outboard? What tire size are you going to run? Do you have coilovers, camber plates or bigger brakes? Will the car be driven on the street, trackday or rally special?
The answer is almost always that it depends on the setup. An aggressive flush fitment that works on a show car may not work when the car compresses hard in a corner. A safe street setup may feel too cautious for someone chasing wider tracks and better chassis support. The right choice is the one that matches how the car will actually be used.
If you are in the middle of a build and want to avoid buying on a whim, it is worth taking the measurements properly before ordering. At https://houseofmotorsport.com you will find parts for builds where the whole counts – not just what it looks like in the product image.
Once you can calculate ET yourself, the whole game changes. Then you don't buy rims based on hope and forum rumors. You buy the right part for the right car, with the right margins for your particular driving. That's how you build faster, smarter and with less mess in the garage.
