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Know how to measure your car’s wheel size

  • Cars Explained
  • Xavier Sabastian
  • 8 minutes

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Knowing how to measure your car’s wheel size might not seem important, but knowing how to go about it is handy knowledge. Installing the right tires on your car is crucial to its overall performance and safety. Your tire selection should be based on your driving conditions and how you desire your car to respond and handle.¬†

When it comes time to purchase tires, you must grasp how to read tire size to make the best-informed decision. Fortunately, calculating the proper tire size for your car is rather straightforward. Either your owner’s manual or a location on the car itself contains all the pertinent information you require.

How to find your car's wheel size

In addition to actual physical dimensions such as width and diameter, there are extra elements to consider when determining the correct tire size. In addition to comprehending concepts such as load index and speed rating, which affect your car’s overall capabilities, it is necessary to know how to measure car wheel sizes.

How to find your car’s wheel size

Regardless of your car, it should be simple to determine the manufacturer-recommended tire size. The best place to begin your search is with the owner’s manual.

If you cannot find the handbook, you can also obtain a tire fit guide in the following locations:

  • The driver’s side door jamb
  • Inside your glove box door
  • Within the fuel tank cover

However, you must decipher a series of numbers and letters regardless of where you locate your tire size.¬†Don’t worry if it initially appears confusing. Each letter and number has a clear purpose. But, first, let’s examine how to read tire sizes.

Basics to know to measure your car wheel’s size

Let’s examine the meaning of each number and letter in your tire size data in the order that they appear. This tire size will serve as an example:¬†P205 /55R16_91v.

Difference between ‘P’ and ‘LT’

For most cars, the letter “P” appears before the number sequence: P205 /55R16_91v. The “P” stands for “P-metric,” which is a Tire and Rim Association mark for “passenger car” tires. The letter “P” indicates that the tire was built primarily for passenger vehicles, such as sedans, minivans, SUVs, and other light-duty pickup trucks.

Similarly, “T” represents your spare tire and stands for “temporary.”¬†

Width

The first value in your tire size information indicates the width of the correct tires for your vehicle, in millimeters: P205 /55R16_91v.

Tire width always refers to the distance between adjacent sidewalls.¬†Consequently, a tire marked “P205” is intended for a passenger car and has a nominal width of 205 millimeters.

Aspect Ratio

The number that follows the slash denotes the tire’s aspect ratio, which essentially indicates the tire’s profile height: P205 /55R16_91v. Aspect ratios are presented as a percentage. The aspect ratio is calculated by dividing the sidewall height by the tire’s width. For example, a tire with an aspect ratio of 55 has a height of 55% of its width.

Construction

After the aspect ratio is a letter indicating the manufacturing of the tire: P205 /55R16_91v. “R” relates to the tire’s internal structure and provides a general sense of stability.

Two types of sidewall construction may be visible on a tire:

  • R – Radial Ply
  • D – Diagonal or Bias

Since radial tires have been the industry standard for over 40 years, you will always find the letter “R” on tires. This is because radial construction indicates that the internal ply cords of the tire are aligned radially (at 90 degrees to the centerline of the tread).

Rim Diameter

The following number is the diameter of the wheel. For example, a tire with the size P205 /55R16_91v would fit a 16-inch diameter rim.

Note that if you decide to change the size of your rims, you will also need new tires that are compatible with the new size. For example, tires made for a 16-inch wheel diameter cannot be stretched to fit on 18-inch wheels.

Load Index

Your tire’s load index is the next quantity needed to interpret tire size numbers. The load index indicates how much weight, in pounds, a fully inflated tire can support P205 /55R16_91v.

It is referred to as the load “index” because it does not tell us the exact number of pounds the tire can support, at least not on its own. The number instead relates to a specific load capacity indicated in an index.¬†

Starting with one and ending with 150, the load index values denote carrying capacities ranging from 99 to 7385 lb. Additionally, this number may not be present on all tires. This is because the load index does not need to be printed on tires by law. Consult the owner’s manual if your tires do not contain a load index measurement.

Speed Rating

The final number in a succession of tire sizes is the speed rating, represented by a letter: P205 /55R16_91v. In the same way that your load index number refers to a specific load, your speed rating letter corresponds to a specific speed capacity as determined by a standardized laboratory test.

A tire with a speed rating of “S” is rated for up to 112 mph, whereas a tire with a speed rating of “R” is rated for up to 106 mph. Remember that this is not the suggested cruising speed but the tire’s maximum speed rating.¬†

You must always adhere to posted speed restrictions on streets.

Here are the most prevalent letters for speed ratings and their maximum speeds:

  • Q: 99 mph
  • R: 106 mph
  • S: 112 mph
  • T: 118 mph
  • U: 124 mph
  • H: 130 mph
  • V: 149 mph
  • W: 168 mph
  • Y: 186 mph
  • ZR: 186+ mph

Generally, tires with greater speed ratings provide superior handling capability. Therefore, replacement tires must have the same or a greater speed rating to retain a car’s top speed. If a car has tires with varying speed ratings, the “slowest” tire’s speed rating determines the vehicle’s maximum speed.

Like the load index, the speed rating is not required to be displayed on all tires by law. However, if your tires do not display the speed rating, the owner’s handbook will include the pertinent information for the stock tire type for your car.

Severe Snow Conditions

Some tires may include “M+S” or “M/S” after the primary tire size sequence. This shows that the tire can withstand snow and mud conditions. This marking is typical on radial or “R”-type tires.

USDOT Tire Identification Number

You may see another set of letters and numbers beginning with “DOT” on the sidewall of your tire. This does not indicate the size of the tire but rather that it meets Federal Motor Vehicle Safety Standards. Following the tire identification number is the abbreviation for the Department of Transportation.

The first two letters or digits following DOT indicate the tire’s manufacturing factory, while the last four numbers indicate the week and year of production.¬†

Basics to know to measure your car wheel's size

How to determine the correct alloy wheel size for your car?

To determine the size of your wheels, you must first measure the actual radius of your tire. Then, examine the wheel itself to determine the width of your wheels. Finally, the tire size should be stamped on the exterior, the inside, or the back of the spokes of every alloy wheel.

Can a car’s wheel size be measured using rims?

In the case of a size such as 4.80-12, 4.80 represents the section width in inches. The final number shows the diameter of the wheel, comparable to tire size notation.

Is it ok to put a bigger size of wheel on your car?

While it is possible to alter the size of your tires, it is typically advisable to remain with the size advised by the manufacturer for optimal performance. 

Increasing the size of your tires may increase your car’s ground clearance, but it may also impair your vehicle’s handling, drivetrain performance, and gas mileage.¬†Consider the changes to crucial metrics, such as the load index and speed rating, and how they may affect the capabilities of your car if you opt to raise tire size.

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via GIPHY

Bottom Line

Once you understand how to read the numbers on tires, you can easily filter down your tire options by size.¬†When deciding wheel size, it has become abundantly evident that one cannot simply take a stab in the dark. Whether you’re seeking a staggered arrangement or need to replace broken OEM rims, fitment will be a problem without the proper measurements.

And while you could use the sizes stamped on the back of your rim’s hub, backspacing and offset ‚ÄĒ which are equally crucial ‚ÄĒ are not usually specified. Initially, it may appear challenging, but it becomes considerably less difficult after a couple of attempts.


Here’s more info on¬†top-rated airport parking,¬†the best parking spots in your city, affordable¬†auto insurance, easy¬†Auto refinance, and¬†high-quality car washes near you.


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