Calculate Your Tire Size

Why You Should Calculate Your Tire Size

Have you ever thought why your car comes with four tires of a specific size, and why it fits perfectly under the fenders? You may have seen a lot of oddball variants on the road - cars that ride on high wheels, smaller wheels almost touching the floor, and tractors with different wheels at the front and back. What would happen if you changed any one or two or all four of your tires? Would it make your ride smoother, faster, or more bumpy and painful? Answer to all that lies in the tire’s size. So you have to calculate the right tire size to get the comfort and performance you want. 

Table Of Contents

1. How To Calculate Tire Size?

2. Importance Of Tire Size Calculations

        2.1. Are All The Tire Size Calculations Necessary?

        2.2. Why Should You Calculate Tire Revolutions Per Mile?

3. Changing The Tire Size

        3.1. Can You Change Tire Sizes?

        3.2. Effects Of A Wider Tire

        3.3. Effects Of Change In Tire Profile

        3.4. Effects Of Having Different Tire Sizes

        3.5. Should You Choose Different Tire Size?

4. How To Choose A Set Of New Tires For Your Car?

5. FAQs

1. How To Calculate Tire Size?

There are many possibilities and choices for metric tires. Therefore, the tire size calculation holds good importance. The calculations provide you with diameter, width, sidewall, circumference, and revolutions per mile using the standard tire data.

Metric tires are rather difficult to read than the conventional inches measurement. On that account, this calculation helps to convert given data into a much easily readable format for you. 

To calculate metric tire diameter in inches:

The sidewall or aspect ratio is used here to determine the tire diameter. 

Sidewall height of the tire = [(First Number) x (Second Number/100)] / 25.4         {1 inch= 25.4 mm]

                                              = [(205) x (65/100)] / 25.4

                                              = 5.24 inches 

Now, to calculate the diameter of the tire, you will need to include another parameter, the wheel diameter, which is the number after the letter ‘R’ in the standard specification. 

For example, in the tire specification, 205/65R16, 16 is the wheel diameter. However, the wheel diameter is already in inches, which eliminates the requirement to convert the value.

Tire Diameter Calculator = {Sidewall Height} x 2 + (Wheel Diameter)   [Sidewall height is  present on either side of the rim, hence it is multiplied by 2]

                                             = {[(First Number) x (Second Number/100)] / 25.4} x 2 + (Wheel Diameter)

                                             = {[(205) x (65/100)] / 25.4} x 2  +  (16) 

                                             = 26.48 inches   [for a 205/65R16 tire]

To calculate metric tire width in inches:

Width of the tire = (First Number)/ 25.4

                              = 205/25.4 

                              = 8.07 inches

To calculate metric tire circumference in inches:

Circumference of the tire = π x (Diameter of the tire in inches)                       

                                              = 3.14 x (26.84)

                                              = 84.32 inches

To calculate tire revolutions per mile / tire size speed calculator

1 mile = 63360 inches 

Revolutions per mile = 63360 / (Circumference of the tire in inches)

                                     = 63360 / (84.3)

                                     = 752 rev/mile

For a 205/65R16 tire specification, the results are:




26.84 inches


8.07 inches


5.24 inches


84.32 inches


752 rev/mile

2. Importance Of Tire Size Calculations

2.1. Are All The Tire Size Calculations Necessary?

If you do not want to end up with the wrong tire size for your vehicle, then yes these calculations are important. These calculations eliminate the confusion for you to choose between tires. Often the vehicle manufacturers give the limits of the variation in tire sizes in terms of the overall diameter and width (in inches). Every time you consult a tire technician for a new set of tires for the desired characteristic, there is a good chance of you being told the dimensions in terms of the overall measurements rather than the standard tire size format. Here is where the tire size calculations come in very handy and help you ease off the confusion. 

2.2. Why Should You Calculate Tire Revolutions/Mile?

An interesting fact that you may note, if you run tires with a larger tire diameter than the stock tires, the actual speed at which you are travelling is greater than what is shown on the speedometer. The foremost reason is, larger diameter tires have a larger circumference than the stock ones. Therefore, the distance your vehicle has travelled per revolution is greater. The speedometer functions based on revolution and not on the distance your vehicle has covered. Thus, you are travelling faster than what your speedometer says when you have larger than stock tires running. Likewise, you will be travelling slower than what the speedometer says when you have smaller than stock tires. 

3. Changing The Tire Size

3.1. What Happens If I Change My Car Tire Sizes?

This could be a possible point of uncertainty for you when you’re out in the market looking for a fresh set of tires. Assuming that the rim/wheel sizes are kept the same as stock, changing tire sizes would mean two things, a change in the aspect ratio and a change in the tire width. These two factors affect the driving dynamics of your vehicle. If you reduce the aspect ratio or sidewall height of the tire, the reduction in rubber provides a firmer ride which aids in cornering and increases handling characteristics. A similar effect is gained with an increase in tire width. However, the drawback to such alterations is that you get a harsher ride quality and wider tires also have the tendency to hydroplane which limits their wet and snow traction capabilities. 

It is always recommended to change the wheel diameter as you reduce or increase the sidewall height of the tire. if you change the tire sidewall height without changing the wheel diameter, the overall tire diameter will increase or decrease from the original diameter. Altering the overall tire diameter will result in speedometer inaccuracy and also affect other tire diameter-dependent systems like ABS, traction control and stability control. 

The tire and wheel width are interdependent. So, a maximum variation of 20 mm is allowed when increasing or decreasing the tire width in regards to the actual wheel width. Variating beyond that requires you to also change the wheel with appropriately sized wheels. 

3.2. Are Wider Tires Better For My Car?

Wider tires always translate to better traction. This could be a differentiator for a performance-focused vehicle. However, you may ask, are wider tires better for day-to-day commuter vehicles? While you may be very pleased to know about the increased traction on offer, certain factors are not so pleasant. You are more likely to experience tram lining, also known as nibbling while running tires that are wider than stock. Basically, that means the wider tires have a higher tendency to follow the grooves and ridges in the road or the contours of the road surface without the driver’s input. An increased contact patch also means a significant increase in the steering effort. The larger size of the tire also means added unsprung mass and the rolling resistance of wider tires is also higher. This drastically reduces the fuel economy. Wider tires are also less effective on wet and snow-covered roads. Due to the larger and more horizontal contact patch, the tires do not expel water or snow effectively and are prone to hydroplaning. Therefore wider tires are primarily best for larger and racing focussed vehicles. So unless you have a gigantic horsepower generator under your car’s hood, it is best to stick to the factory recommended tire width. 

3.3. Can I Downsize My Car Tires?

It is not often that you see low profile tires, meaning tires with low aspect ratio on normal passenger cars. Those shoes are meant for high-performance purposes. On the opposite end of the spectrum, high-profile tires are majorly used on heavy-duty trucks and for off-road purposes. You may argue that suspension absorbs all the shocks due to the vehicle-road interaction, tires also have a fair share in absorbing the road shocks. While running low profile tires, you may observe the lower stance it provides, which is often craved for among auto enthusiasts. There is something else that you would not miss. The ride becomes considerably harsh, which is not desirable considering you would not be pushing your vehicle to the limits in a normal driving scenario. A lower profile renders low sidewall flexing, which is the primary reason they provide high stability while cornering and also at high speeds. 

3.4. What Are The Effects Of Having Different Tire Sizes On The Same Car?

If your vehicle is rear-wheel driven or front-wheel driven, until both the front tires are the same and the rear tires are the same, the suspension geometry will remain unchanged for most of the time which translates to your vehicle operating with no inconsistencies. 

While you may get away with having different sizes at the front and the rear in a two-wheel-drive vehicle, it can cause major problems in a four-wheel-drive vehicle. You could damage the differential unless it is designed for it

This is also a reason that you are not allowed to drive over 80 km/h on a space saver wheel (or Stepney or spare wheel)  in the event of a tire replacement because of a puncture. By mounting on a spare wheel, either side of the vehicle is no more the same, causing an imbalance. This inconsistency pulls the vehicle to one side making it difficult to handle and therefore it is not advisable to drive at higher speeds.

3.5. Should I Choose A Different Tire Size?

As subjective as this question is, it was always good to be aware of the limits of your vehicle and the characteristics you desire out of it. Changing the tires sizes can significantly influence your vehicle’s driving characteristics. These enhanced driving characteristics do come at a certain cost. In short, it is a trade-off between comfort and performance. To what extent do you want to trade-off is what will help you decide the right tire size for your vehicle. You can stick to the stock specification of the tires if you feel there the vehicle fits your needs and also when you are less educated about the effects of tire size changes. It is always better to be safe than sorry. 

4. How Do I Choose A Set Of New Tires For My Car?

You would have seen a series of numbers and sometimes letters on the sidewalls of the tire other than the manufacturers’ names, which made no sense to you. But this is precisely the information that you need to know about the tires that fit your car. Here’s a sample of what it might look like. However, there could be a few 

P205/65R16 90H 

  • P-`Type of tire
  • 205 - Width of the tire across the tread in millimetres
  • 65 - Aspect ratio of the sidewall compared to the width of the tire
  • R - Radial construction
  • 16 - Diameter of the rim in inches
  • 90 - Tire's load rating
  • H - Tire's speed rating

Tire Type:

‘P’ suggests that your tire is a passenger tire. If it starts with ‘LT and not ‘P’, then it is a Light Truck tire.  The letter ‘P’ in front of the numbers stands for P-metric tire, referring to certain standards in the United States. If there’s no letter mentioned, it suggests the tire is Euro-metric. There could be a possible difference between P-Metric and Euro-Metric tires in terms of load capacity. 

Tire Width:

The first three-digit number represents the tire width in millimetres from sidewall to sidewall. Here, the tire width is 205 mm. This could vary from vehicle to vehicle. 

Aspect Ratio:

The two-digit number after the slash is known as the aspect ratio. It is the ratio of the height of the tire’s cross-section to its width. In simpler terms, it's the height of the sidewall measured from the wheel rim to top of the tread, expressed as a percentage of tire width. Here, 65 represents the aspect ratio of the tire, meaning the height of the sidewall is 65% of the tire width. The more the aspect ratio, the more is the sidewall height. 

Tire Construction:

The letter ‘R’ following the aspect ratio represents the radial construction of the tires. There are basically 3 types of tire construction, R stands for radial construction. B means belted bias and D stands for diagonal bias construction. The construction of the layers beneath the portion of the tire visibly differs among them and is used for different purposes.

Wheel Diameter:

Following the letter representing the tire construction, the two-digit number is the diameter of the wheel that the tire is intended to fit in inches. The number 16 here suggests a wheel of 16-inch diameter. 

Load Index:

Post wheel diameter, after a certain gap, is a two-digit number that shows the maximum load a single tire can support. However, it is not the direct representation of the load; the numbers have to be compared against a load index chart from the manufacturer to know the exact loading capacity in LBS or KG. The number 90, in this case, equals 1323 lbs of maximum load capacity of each tire. 

Speed Rating:

This is an alphabetic representation of the maximum speed that you can go on that set of tires. For knowing the exact maximum speed, the alphabet has to be compared to a speed rating chart. ‘H’ stands for 130 mph or 210 km/h of maximum speed capability. 

Load IndexLBSKgLoad IndexLBSKg



RatingMaximum  SpeedMaximum Speed


Where Is The Tire Size Mentioned?

Apart from the markings on the tire, the tire size can also be identified at two other places in your car. 

  • The vehicle’s owner manual has the tire size information prescribed by the manufacturer.
  • The information sticker on the driver’s  door jamb has the ideal tire size mentioned on it.

This also contains information about the optimum tire pressure that has to be maintained for the best ride quality.




255/45 R18 103H


290 KPA, 48 PSI


330 KPA, 48 PSI



255/45 R18 103H

330 KPA, 48 PSI

5. FAQs

Q) Are Wheels The Same As Tires?

Tires and wheels are not the same. Yes, they are interdependent, but they are not the same. The part that is made of rubber, the part in contact with the road surface, is known as the tire. Wheels, fondly known as rims, are the round metal parts that are connected to the hub and an axle. Wheels are usually constructed in Steel or Aluminium Alloy. The tires are mounted on the wheels.

Q) Should I Change Both Tires When Only One Is Worn Out?

Will you wear a pair of shoes if one of the shoes is worn out? No, Right? The same is with your vehicle tires. Ideally, the wear on tires should be fairly even. If this is not the case, it is indicative of tire pressure difference or misalignment. To have a consistent ride quality, it is better to get both the tires replaced. However, it is not always worthy of getting both of them replaced, especially when the other tire is in a fairly new condition. If both are fairly worn, the difference in the overall diameter would be significant, which is not desirable for stable driving dynamics of a vehicle. Hence, you should change both of the tires in that case. 


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Dominic Lussier
Lead Mechanic
Passionate about the inner workings of motor vehicles there is absolutely nothing that he won’t take apart. He loves getting his hands dirty so you won’t have too !!!! Helps empower DYI’ers to fix their own vehicles with the help of easy-to-read articles so things just stay simple.