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How to Determine Car Tire Size

How to Determine Car Tire Size

When your car tires complete the manufacturer’s recommended mileage, they are, in most cases, worn out. Well, they have served you, and it’s time to know how to determine car tire size again to continue enjoying your ride.

Many car owners don’t know choosing the wrong car tire size messes up your ride. More so, you end up overspending on tires as they wear out faster.

With that information, I would like to share with you challenges you would face for wrong car tire choice;

  • A pull in the steering wheel.
  • The tires rub against the vehicle’s body and suspension.
  • Noisy ride as tires rub against the suspension.
  • Reduced vehicle performance and poor gas mileage
  • Unstable ride

To avoid the above challenges, choose the right tire size. You’ll have done yourself a favour by avoiding unnecessary vehicle repair, tire replacement, and you’ll enjoy your ride.

A Key Guide on How to Determine Car Tire Size

Although many drivers choose various tire sizes, designs, and from different manufacturers, it’s good to consider the tire size specifications from your car manufacturer.

Car manufacturers perform various tests to determine the car tire sizes that the engine can power with ease, maintain stability and safety.

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As such, I’ll share with you possible locations in your car you can find tire size specifications;

  • On the driver’s side doorjamb
  • A sticker on the glove compartment box
  • On the Gas tank hatch door
  • Vehicle owner’s manual

Depending on your manufacturer, you’ll find the car tire size specifications in one of the listed areas or documents.

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Understanding the Car Tire Size Numbers

On every tire sidewall, you’ll see stamped numbers or codes. Surprisingly, many drivers don’t understand these numbers describe the tire size and its capability.

However, I’ll easily educate you on reading the numbers and comparing them with your car manufacturer’s specifications. You’ll see codes like;

  • Example: P 225 / 50 R 17 98 H
  • Example: LT 245 / 75 R 18 85 S
  • Example: ST 255  / 75 R 22 110 F
  • Example: T 215 / 45 R 14 65 P

Every single or set of numbers on codes carries certain information. In the table below, I’ll explain their meanings. Let’s use the first example written vertically.

pTire TypeThe type of vehicle that can use the tire, either passenger or commercial.
225Tire WidthThe tire width in millimetres (MM)
50Aspect RatioHeight of the tire from the rim edge to the top.
RConstruction TypeDenotes the composition of the tire by materials used.
17Wheel DiameterThe diameter of the tire rim is in inches or centimetres.
98Load IndexThe weight a tire can handle.
HSpeed RatingThe maximum speed the tire can handle.

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 The Tire Type

The first letter of the code represents the tire type. However, on the above examples, you’ll note they have different letters at the start like P, LT, ST, and T. They all represent the tire type. How?

  • P- stands for P-metric. Meaning it’s a passenger vehicle tire. It’s generally manufactured for vehicles with lower cargo like cars, small pick-up trucks, minivans, crossovers, and SUVs.
  • LT- stands for LT-metric, which means light truck tires manufactured for heavy load trucks and towing trailers.
  • ST- the ST-metric represents special trailers. The tire is purposely manufactured for unpowered vehicles with heavy loads.
  • T- It simply means temporally. They are manufactured as spare tires for light commercial and passenger vehicles.

 The Tire Width

The second three digits represent the tire width measured in millimetres. On the upper surface that comes into contact with the surface when rolling, the width is measured from one side of the tire to the other.

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Aspect Ratio

After the slash separating the tire width is the aspect ratio number. The aspect ratio is the two digits that represent the tire’s height from the rim edge to the top. It’s measured in percentage, depending on the tire width.

For instance, in the first example of the codes, the aspect ratio is 50. Meaning the length of the tire from the rim edge to the top is 50% of the tire width.

Construction Type

Construction type is the internal composition of the tire. It consists of strong cords of polyester, fabric, and steel materials coated with rubber.

However, you may come across a construction type denoted by a different letter from “R” as in the examples. There are two types;

  • R- Represents radial ply, meaning the tire cords are laid parallel to the rolling direction of the wheel.

The R-types are common in the modern market due to their durability and the ability to grip. After various testing, the R-types have proven to be highly friction resistant.

  • B or D- it stands for Bias or Diagonal ply. The cords in these tires are crisscrossed to the sides of the tire.

Wheel Diameter

The wheel diameter is the measurement of the rim from one edge to the other in centimetres or inches. It represents the size of the rim the tire can fit. For instance, in the first example, the rim size is 17.

 Load Index

The second last three or two digits denote the maximum weight in pounds or kilograms the tire can support when fully inflated.

In the first example, the load capacity is represented by 98, which according to the Tire and Rim Association charts it denotes 1653 pounds.

Speed Rating

The speed rating is the maximum speed a tire can handle despite standardized tests’ friction and load capacity. Higher-speed tires dictate an increase in performance.

Taking the first example, the letter H represents the speed rating. According to the speed rating chart, it denotes a speed of 130 miles per hour.

To understand more about load index and speed rating codes, you can read your tire user manual.

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Other Stamps You Would Find On the Tire Side Wall

There are several other codes you would find on your tire sidewall;

  • M+S or M/S code means the tire can handle mud or snow conditions. These tires are made of special rubber, unique tread patterns, and large gripping edges.
    • DOT Quality Symbol means Department of Transportation that shows the tire meets US federal vehicle safety standards. After the DOT symbol, you’ll notice a serial number with the first letter representing the manufacturer and the last four codes representing the week and year of manufacturing.
    • Uniform Tire Quality Grading (UTQG) – the US transport department uses the code to assure the users of quality tires based on tread wear, traction, and temperature.

FAQs on Tire Sizes

What is the Most Common Rim Size for Cars?

The most commonly used rims for standard road cars are sizes 14 to 19. However, several models like SUVs, sports cars, and luxury cars may have bigger rims.

What Are Crucial Factors to Consider When Choosing Rims?

There are several factors to consider when choosing your car tire rims;
– Fake rims in the market.
– Car manufacturer’s recommendations
– Wheel offset and width affect car performance too.

How Does Car Wheel Size Affect Performance?

Car wheel size affects the performance in various ways. For example, larger wheels cover a large road surface, increasing gravitational resistance. As a result, the engine strains cause slow acceleration and poor gas mileage. On the other hand, small wheels have less ground clearance that may cause difficulty when driving on a rough road.

However, as earlier stated, it’s wise to read the vehicle owner’s manual to determine the car tire size.

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