How to work out what wheel size to buy
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Team Driftworks
- Guides & Featured Products
- 13 Aug 2024
- Reading time: 6 minutes
You've finally found a style of wheel you like the look of, but it's always wise to double-check how a new wheel size will compare to the one you already have fitted.
We're going to look at an example, showing how to assess the width and offset you need to get the fitment you want.
This is easy to do and should help you get a good wheel fitment every time.
If you're not sure about what wheel offset is you should probably check out our article How do you measure wheel offset? first.
You will need the following:
1x Alloy wheel (you'll need to know the diameter, width, and offset)
1x Measuring tape/ruler
1x Internet connection
For this article, we are going to use a hypothetical Nissan and a 17"x7.5 wide OEM wheel which has an offset of ET35 to give a reference point.
We want to work out what wheel size we need to get good wheel fitment with our new over fenders which are 35mm wider than the standard arches.
Go to the car in question and stick the reference OEM wheel on with your new over fender at least test fitted (we would always recommend doing this with the arch test fitted as you never know if fibreglass has shrunk or expanded in the mould and you don't want to waste your money on the wrong wheels.) Ideally, your reference wheel will have a tyre fitted, so you can sit the full weight of the car on it and measure it at your chosen ride height. If there's no tyre, you can either sit the wheel gently onto a block of wood or jack up the lower arm.
With the wheel now fitted and at a suitable height, measure how much further in or out the wheel would need to be from its current position.
Be it 15mm, 30mm, 250mm, whatever it is to make you happy and ensure the wheel won't contact the suspension and chassis on the inner edge, or the wheel arch and bumper on the outer edge.
For the sake of argument, we are going to say we need them to be 65mm further out than the current 17x7.5 ET35. (30mm to get flush to where an OEM arch would be, 35mm further to get flush with the new arch.)
1. Now head over to our offset tool at www.willtheyfit.com and input the current OEM wheel size we're using in this example of 17x7.5 ET35. Don't worry about the tyre width and profile for now, add a 1 to both fields.
2. Try some wheel sizes for a starting point like 18x10.5 rear wheels, such as the Work Emotion CR Kiwami 18x10.5 ET15.
Hit the 'CALCULATE' button and this will give you some measurements.
3. First, we get details about your speedo, etc. You can ignore those for now as they will mostly be dictated by your choice of tyre size.
Next is the important bit.
Compared to your existing wheel size, this new wheel size will have an inner rim which is 18.1mm closer to the suspension strut. The outer rim will poke out 58.1mm more than before.
In theory, you would now get your measuring tape and physically check these measurements on the car.
If you have 18.1mm of clearance to the coilover from the current wheel you're ok.
Does the wheel stick out 58.1mm more than it currently does? Is it a problem? If not, then it's all good.
The ET15 offset will mean you are not quite at the 65mm difference we need to be flush with the fender. As the CR Kiwami is a 1-piece wheel, there is no way to make the wheel sit at the 6.9mm needed, so in this case, you would use a slip-on 5mm spacer.
With a 2-piece wheel, you can request the exact offset needed which would be 18x10.5 ET8 - you would now be in perfect wheel fitment territory.
You can use this method to work out any fitment you like, it just takes some experimenting - remember to measure, and double-check your numbers every time.
A few things to remember:
- If the wheels are too close to the inside of the suspension, reduce the offset value or wheel width.
- When the wheels poke too far from the arch, raise the offset value or reduce the wheel width.
- With wheels that are too far from the suspension and hitting the inner arch or chassis when turning left to right, increase the offset.
- For wheels which don't come out far enough then lower the offset.
- If you can't get the exact offset you want in your chosen wheel, or you've found some specific wheels to buy, but their offset is too high, you can always lower the offset by adding Wheel Spacers
Most importantly, check before you buy. Whether you are looking at new or second-hand wheels, if you do these basic checks beforehand you should get a good idea of which wheel size will work for you.
If you have any questions, please get in touch with our friendly team of wheel specialists at [email protected] or call 0121 792 2000 and they'll discuss with you everything you need to know about getting perfect wheel fitment on your car.
We're here to give you the most up-to-date knowledge on frequently searched topics gained from years of first-hand experience modifying our cars. We'll always aim to give you the answers straight without adding over complicated, unnecessary detail.
