Wheel Offsets and Widths Explained

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Wheel Offsets and Widths Explained

Postby Danny_Boy » Tue May 17, 2016 8:54 pm

Hello people,

I'm bored at work so thought i would help write out a thread for people to read when looking for wheels to fit their cars.

Loads of people do not have any idea what J's and ET's are when it comes to wheels, well, read below!

this is the width of the wheel from inner to outer rim in inches. so an 18x8J is 18 inches in diameter of course and 8 inches from inner to outer rim.


this is what causes people a LOT of confusion! the ET is basically the distance the mounting surface is from the centre of the wheel. So on an 8J wheel, imagine there is a line 4 inches inbetween the wheel yes? Right, now the ET is the distance in millimetres that the hub sits from this centre line.

POSITIVE offset like ET35 means the mating surface of the wheel sits 35 millimetres closer to the OUTSIDE of the wheel. So effectively more of the wheel goes over the hub and sits in the arch closer to the suspension strut.
So the lower the number towards ZERO the more the wheel will stick out, the HIGHER the number, the more it will sit INSIDE the arch towards the suspension.

NEGETIVE offset means the hub mating surface is closer to the INSIDE of the wheel so this means the wheels stick out further. so ET-35 means the hub mating surface sits 35 millimetres closer to the INSIDE of the wheel.


NOW....... a combination of the two!

We shall use the same wheel OFFSET to explain

18x8J ET40, so 8 inches wide with a hub mating surface thats sits 40mm closer to the outside of the wheel.

If you wanted a wider wheel so an 18x8.5J ET40 wheel, we need to take into consideration that the wheel is half an inch (12.7mm) wider than the 8J. BUT because the wheel is the exact same offset, it means that the extra width is split EQUALLY each side, so the wheel will sit further in the arch towards the suspension strut 6.3mm and further towards the arch itself 6.3mm.


what about an 18x8.5J et35....

Well this wheel is still 12.7mm wider than our last wheel, but because the hub mating surface sits 5mm closer to the hub, it automatically means the wheel will stick out 5mm more. So lets apply this:

We have the fact the wheel is 12.7mm wider
We have the fact there is a 5mm offset difference.

SO... 6.3mm wider each side, then add the 5mm offset difference to the OUTSIDE as it stick out further, so we have a wheel that will stick out 11.3mm MORE than out 18x8j et40 wheel and will stick in only 1.3mm closer the the inside/strut as you minus the 5mm offset difference.

Make sense?

If not, go to Willtheyfit.com and let that do it for you :lol: :lol: :lol: :lol:
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Re: Wheel Offsets and Widths Explained

Postby Floydian » Wed May 18, 2016 12:14 am

Wow thanks Man


I never could fully understand what was ET, I just knew the smaller the number the more wheels stick out.

The forum should really have a like button for all the effort but it doesn't.
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Re: Wheel Offsets and Widths Explained

Postby rjwojcik » Wed May 18, 2016 7:06 am

Good post :thumbs:
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Re: Wheel Offsets and Widths Explained

Postby weasley » Wed May 18, 2016 8:11 am

What isn't mentioned is that offset is about more than just "will it fit?" and "will it look sick, innit?". The offset is part of the overall steering and suspension geometry and changing it can impact both, which can lead to various issues; perhaps bush wear, tyre wear, joint wear, tramlining etc. One dramatic possibility is that, in the event of a blowout, the effect on the steering can be unpredictable. Such factors are taken into account by the chassis designer.
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Re: Wheel Offsets and Widths Explained

Postby marco_polo » Wed May 18, 2016 8:18 am


Try to stick within 1/4" of standard offsets where possible, or it will effect your scrub radius.
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Re: Wheel Offsets and Widths Explained

Postby Twenny » Wed May 18, 2016 9:23 am

Nice write up Danny boy

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Re: Wheel Offsets and Widths Explained

Postby roundasapound » Sat May 28, 2016 9:36 am

Thank you. I'm going to read this about 10x before it all sinks in :shock: .. And still ask "Will these wheels fit?" :lol:
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Re: Wheel Offsets and Widths Explained

Postby atnem » Sat Jul 23, 2016 12:19 pm

Thanks for the post I've been curious about this for a while!

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