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Tire Size and Wheel Offset – Decisions, Decisions Blog Feature
JP Alonso

By: JP Alonso on January 27th, 2016

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Tire Size and Wheel Offset – Decisions, Decisions


Every day you wake up, you think about your car.  You spend time on Instagram liking photos.  You check your Facebook news feed and you see the endless stream of funny videos, posts you wish you didn’t see, and car pictures.  Every once in a while you come across that car that makes you want that exact same setup or something close to it.  But how do you achieve it?  Who’s car is that?  Should I message them?  Where can I find their build specs?  You must find out what that setup is.  It can be a spoiler, a vinyl wrap, a headlight setup, a turbo kit, or anything else.  It seems more often than not, it’s about the wheel setup.  How did you achieve that perfect offset and stance?  What size are the wheels, do the tires rub on the fenders?  The questions persist.

The truth is, you don’t need to ask them or research, unless you want to see what a specific wheel and tire combo looks like on a car.  First off, you know what your style is and what’s acceptable to you and your car.  Outside of that, you just need to get down to the details.  It’s not rocket science but you need to understand a few things about wheel size, tire size, and how they work together.   

In this blog, we’re not going to write about how to fit a 195 wide tire over a 9 inch wheel (if you don’t know what that means, keep reading).  We respect the style and individuality of those that choose that route but understand that we are talking about what it takes to create an aggressive or sporty stance while maintaining full drive-ability and SAFETY.  I would venture to say that although the aggressively cambered and fitted vehicles are constantly in your social media feed and attend many shows, the majority of enthusiasts care about keeping a functional stance that they can drive on with minimal issues.

The first thing to consider is the new tire and wheel size.  The tire and wheel placement have the largest affect on the stance of a vehicle.  You can have a car lowered to the max and if the wheel sits too far inside the wheel well and it’s too small, it still looks like crap (compared to the unwritten rules today).  You can have a wheel that fits perfectly in the wheel well and flush with the fender but if it’s on stock suspension, someone might think you’re on the way to a monster truck event and you’re Grave Digger’s biggest fan.

Here’s how you get your sizing right.  Take the stock wheel and tire dimensions and get your calculator out.  I’m not going to teach you math, but I’m going to give you an example.  From there, you can plug in what you want to do with your own individual sizing requirements.

Tire Measurement – To find the size of the tire, look at the sidewall.  Every tire has this built in so it’s clear.  It’s like reading the tag on a shirt.  There is tire tread width, aspect ratio (the percentage of sidewall height in relation to tread width), and wheel diameter.  There is more information here but the only three we are using are what is shown.


Wheel Measurement – Wheels have multiple dimensions and measurements that are important to consider.  There’s bolt pattern, center hub bore, wheel diameter, wheel width, and offset.  Find out what wheel specs the car currently has on it.  If they are stock, use this website.  It’s an awesome site that has tons of information on stock wheel sizes.  You’ll need to know what size your wheel is, as a reference point, before you determine what you want.

The bolt pattern is what it is.  Find what it is for your car and make sure the wheel you want has the same. 

The center bore is typically not that important as many wheel manufacturers will include a hubcentric ring that will cover the distance between the bore on the car and the wheel bore.  Sometimes, it’s a perfect fit and it’s not necessary to have the ring.

The wheel width needs to at least be known to start with, along with the wheel offset.  We’ll dive into how to use this information later to determine what size you want.

The wheel diameter is easy to figure out but understand that if you want a larger wheel diameter than stock, it’s going to take up room that the tire needs.  If you’re decreasing the wheel diameter, you’ll generally need a higher profile tire.

Generally speaking, you want to keep the same OVERALL TIRE AND WHEEL HEIGHT as stock for the purposes of any given street car.  Tons of people change this and it’s not a bad thing, you just have to know what to change to make it look decent and perform correctly. 

To measure your overall wheel and tire height, perform the following equation.

Overall_Tire_and_Wheel_Height.pngsidewall height × 2 + wheel diameter = overall height

If this is all new to you, your next question should be how do you calculate sidewall height?  Here you go.

tire tread width × aspect ratio ÷ 100 = sidewall height
tire tread width × aspect ratio in percentage = sidewall height

Let’s say you have a 225/45 R18 tire.  If you don’t know already, 1 inch = roughly 25.4 millimeters.  Because tires use mm as units of measurement, we’re going to use mm in overall height then convert it to inches.


225mm (tread width) × .45 (aspect ratio percentage) x 2 (there are 2 sidewalls to measure in overall height) + 437.2mm (18 inch wheel) = 639.7mm (roughly 25.2 inches) OVERALL TIRE AND WHEEL HEIGHT

If you were to change the tire size to a 245/40 R18, here’s what you’d get.

245mm × .40 × 2 + 437.2mm = 633.2 mm (roughly 24.9”)

This is typically a small enough difference and actually ends up giving you a tiny bit more clearance in the wheel well to fit the tire.  Again, you want to stay as close as you can to the stock height for most street setups.  The further away from factory height you get, the more your speedometer will be inaccurate and you may affect your suspension negatively.  Some tuning devices have the ability to change the tire size in the ECU in order to calculate the new corrected speed output but if that isn’t possible for you, this is something to consider. 

Once you’ve measured your overall tire and wheel height, go to your car and measure the wheel gap as it currently sits.  This is the distance between the top most part of the tire and the fender location directly above that.  Make sure you have plenty of room to fit it in the wheel well.  In theory, if you’re going with the same overall height, it’s going to look the exact same as stock.  At this point, you’ll need to figure out how far out you want the wheel to sit in relation to the fender and how far in the tire and wheel will sit inside the fender.  This is where offset becomes a factor.

Determining your offset

Most people want a wider wheel than what the factory gave the car.  So let’s say you’re increasing your width from an 8” wide wheel to a 10” wide wheel.  How do you know what offset to get?  First off, you need to know what offset and width the car started with as a reference point.  Using the website mentioned above, make note of your wheel size.  Find what your stock width and offset is and then get a calculator out again.  Let’s pretend the wheel size is 18x8 with an offset of +50mm from the factory.

First of all, what does that mean?  The first two numbers are easy.  This is the wheel diameter (18”) by the wheel width (8”).  The 3rd number is the offset and what that translates into is the THE WHEEL MOUNTING SURFACE’S DISTANCE FROM THE CENTER LINE OF THE WHEEL BARREL.  So a +50mm offset means that the wheel’s mounting surface is 50mm TOWARDS THE OUTSIDE OF THE WHEEL (+ is towards the outside of the wheel, - is towards the inside of the wheel).


There are 2 things you need to consider here.  If you’re using the same width as stock, all you need to pay attention to is the offset of the new wheel.  If you are changing the width of the wheel, the offset equation changes completely and you need to reevaluate the offset.  So here you go.

Stock wheel size example – 18x8 +50mm

New wheel size example - 18x8 +40mm

Since the offset is now CLOSER to the center of the wheel than it was before, this PUSHES THE OUTSIDE OF THE WHEEL 10mm FURTHER out from where it was sitting before.  This is also going to push the inside of the wheel 10mm further out.  The entire wheel moves 10mm further out than where it was before.

Now let’s use the same offset measurement but use a wider wheel width.

Stock wheel size example – 18x8 +50mm

New wheel size example – 18x8.5 +50mm

Let’s do some math.  The width has changed.  Now that you are adding width the offset is not an apples to apples comparison.  You need to calculate where the center of the wheel is and then add the offset. 

Stock wheel size

8” x 25.4mm = 203.2mm (wheel width)

203.2mm ÷ 2 = 101.6mm (This is to find the center of the barrel)

101.6mm – 50mm = 51.6mm (from mounting point)

This last part is confusing.  Why wouldn’t you add the offset since it’s a positive offset?  Well, we’re trying to figure out the distance where the outside edge of the wheel sits in the fender compared to where it sits now.  So when you subtract offset from the center measurement of the barrel, this is how much distance there is from the hub to the edge of the wheel.  When the offset is negative, you would then add the offset number instead of subtract because the hub sits on the inner half of the wheel rather than the outer half like the positive offset.  Again, you’re figuring out how far from the hub the outside of the wheel sits as a reference point. 

So let’s go back to the new wheel size

8.5” x 25.4mm = 215.9mm

215.9mm ÷ 2 = 107.95mm (Center of barrel)

107.95mm – 50mm = 57.95mm (from mounting point)

Now the picture is a little clearer.  With the new wheel size, the outer edge of the wheel sits 57.95mm away from the mounting surface or hub.  This is 6.35mm further out then the other wheel we replaced which is about ¼”. 

We’re not done.  Since you changed the width of the wheel, you also changed where the inner part of the wheel sits.  Once you’ve figured out the first equation, all you do is subtract the first number from the total width of the wheel then you have the distance from the hub to the inner edge of the wheel.  Let’s compare both sizes again.


8” x 25.4mm = 203.2mm

203.2mm ÷ 2 = 101.6mm (This is to find the center of the barrel.)

101.6mm – 50mm = 51.6mm (DISTANCE FROM HUB TO OUTER EDGE)

203.2mm – 51.6mm = 151.6mm (DISTANCE FROM HUB TO INNER EDGE)


8.5” x 25.4mm = 215.9mm

215.9mm ÷ 2 = 107.95mm (Center of barrel)

107.95mm – 50mm = 57.95mm (DISTANCE FROM HUB TO OUTER EDGE)

215.9mm – 57.95mm = 157.95mm (DISTANCE FROM HUB TO INNER EDGE)

So when you compare the inner wheel’s distance from the hub from the original wheel to the new wheel, the distance is also greater by 6.35mm.  This means that the inner side of the wheel sits 6.35mm further inside of the wheel well than it did before.  IF THE OFFSET IS THE SAME on a wider wheel, the equation can more easily be done by figuring the distance change once on one edge.  After that, the change is the same on the other edge just like in the previous example.  But look what happens when you change the offset number along with the width.  Let’s pretend we are going to a 10” wide wheel with a more aggressive offset of +35mm. 


8” x 25.4mm = 203.2mm

203.2mm ÷ 2 = 101.6mm (This is to find the center of the barrel.)

101.6mm – 50mm = 51.6mm


10” × 25.4mm = 254mm

254 ÷ 2 = 127mm (Center of Barrel)

127mm – 35mm = 92mm

Now, we have a wheel that sits 40.4mm further out than where the last wheel sat (roughly 1.6 inches).  This is going to give you a much more aggressive fitment than before.  Don’t forget to do the other inner edge equation!  Using the previous equations, this also extends the inner part of the wheel 10.4mm further in, bringing it that much closer to the shock and spring. 

Continue experimenting with different offsets that are available in the wheels you want and plugging the values in the formulas to see where the wheel is going to be positioned.  Ultimately, you need to go to the car and measure the distances on the car to see where the calculated fitments are actually going to end up.  Measure the distance between the edge of the wheel (right where it meets the tire bead, not on the spokes) and the edge of the fender.  Compare the calculations to the actual measured number and that’s how you determine where you want your wheel to sit in relation to your fender.  If you want to figure out the inner and outer edge equations much easier, you can use this very useful online calculator.  All you do is plug in the current wheel size you have and the wheel size you desire and it will give you the measurement of how much further out or in the edges of the new wheel will sit. 

Lastly, make sure you pick a tire that fits properly on the wheel.  In order to get the best and safest use out of the tire, use the following chart in order to choose the right width of tire for the corresponding wheel width you are putting on your car.


With tires, they are not all measured equally.  Different tire manufacturers end up having slightly different patterns and structure of the overall tire which contributes to some minor differences going from tire to tire.  If you’re unsure, try to find someone that has ran the same size setup you are going for and see what their experience has been. 


About JP Alonso

I'm the founder of Edge Autosport and I remember first getting into cars in high school. I read all the magazines, bought a bunch of technical books, and finally got to start wrenching around the age of 19. I really enjoy modding and being able to live out a passion is truly awesome. I wouldn't change a thing.

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