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How to Read Tire Dimension

How to Read Tire Dimension

Reading the side of a tire can be intimidating and at times, may seem like a complex algebraic equation. These numbers and letters each serve a distinct purpose on the fitment and usage of the tire. Using one tire as a primary example, let's learn what each set of letters and numbers stands for and then this process can be applied to any other tire.

The Nitto 555 (275/40-ZR18 103W), is a great performance tire for your Corvette, but what exactly do these numbers mean? First, the 555 represents the tire model, which will vary from brand to brand and also model to model. Something to keep in mind when reading a tire is that manufacturers generally make the model name and number larger then the dimensions, because it’s the name that sells the tire and not the size. Besides being in different size text on the side wall, they are also found in a number of different locations. The brand name and model are centered on the side wall, while the dimensions are closer towards the mounting lip of the tire.

Once the dimensions are located on the tire comes the fun part. This is the (275/40 – ZR18 103W) of the tire we are practicing on. There are 4 different sections of the tire to understand and we will go through in the order that they appear on the tire. So first up is the 275.

The first dimension listed on the tire is the width, in this case 275.(See Picture Below) This and most other dimensions are listed in millimeters. So this Nitto tire is 275mm wide, which when converted into the English unit from metric would be 10.8 inches. When selecting the proper tire for your Corvette remember that it has to fit onto the rims you have so if the width is too wide or too narrow it will not fit properly and will create safety issues.

The second dimension listed is for the side wall height, and is based on an aspect ratio of the tire width. In this case it is 40% of the width, denoted by the 40, which is determined by a simple equation:

Tire Width x Aspect Ratio = Actual Sidewall Height
275mm x .40 = 110mm

Remember that the overall height of your tire must stay relatively the same, otherwise your speedometer and odometer will not read properly. If you are going from a 17" rim to a 18" rim you will want a smaller side wall on your tire to keep the same overall height of your rim and tire combination.

This picture shows the listing of the tires dimensions on a Nitto Tire, other brands may vary slightly in their location

The next set of dimensions is ZR18, this is both a dimension and a tire rating. The ZR is the speed rating where the Z signifies the speed the tire is capable of handling, this will be described in greater detail below. The 18 is the dimension for the height of the rim, obviously you would want to directly match this to your rim size, where the other tire dimensions can be slightly fidgeted to get the performance capabilities you desire.

103W, our last set is not dimensions but ratings for the tire. The numbers indicate the load rating of the tire, this is the amount of vertical pressure the tire can support when properly inflated, while the letter is the speed rating, again.

Now you might be asking yourself, “Why are there two speed ratings?” The reason for this is due to an enhancement of automobiles over the past couple of decades. Where before automobiles were not capable of the high speeds they are today, so when tire manufacturers made up the rating system they figured a Z rating for tires that could withstand speeds over 149mph that would be OK. But then drivers like yourself with your Corvette are left asking how far over 149mph are these tires safe. Tire manufacturers then added a W and Y rating to specify how fast over the Z rating they could go. A chart is provided below with the complete range of speed ratings. The Nitto tire we are deciphering would have a Z speed rating of W then, and it would be capable of handling speeds of up to 168mph.

  • Speed Rating
  • MPH
  • KMH
  • M
  • 81
  • 130
  • N
  • 87
  • 140
  • P
  • 93
  • 150
  • Q
  • 99
  • 160
  • R
  • 106
  • 170
  • S
  • 112
  • 180
  • T
  • 118
  • 190
  • U
  • 124
  • 200
  • H
  • 130
  • 210
  • V
  • 149
  • 240
  • W
  • 168
  • 270
  • Y
  • 186
  • 300
  • (Y)
  • 186+
  • 300+

Back to the load rating for this Nitto tire, it is rated at an index of 103, indicating that this tire when inflated can withstand about 1929 Lbs of vertical pressure. This is more then ample for your Corvette, considering that this is for one of your four tires on the car. Also Corvettes weigh in at about the 3,000 to 3,500lbs mark, so each tire withstanding less then 25% of their load index limit. Again a table is provided below for referencing the range of load indexes.

  • Load Index
  • Lbs.
  • kg
  • 88
  • 1235
  • 560
  • 89
  • 1279
  • 580
  • 90
  • 1323
  • 600
  • 91
  • 1356
  • 615
  • 92
  • 1389
  • 630
  • 93
  • 1433
  • 650
  • 94
  • 1477
  • 670
  • 95
  • 1521
  • 690
  • 96
  • 1565
  • 710
  • 97
  • 1609
  • 730
  • 98
  • 1653
  • 750
  • 99
  • 1709
  • 775
  • 100
  • 1764
  • 800
  • 101
  • 1819
  • 825
  • 102
  • 1874
  • 850
  • 103
  • 1929
  • 875
  • 104
  • 1984
  • 900
  • 105
  • 2039
  • 925

Many times when buying new tires they are just a direct replacement of the old tire retaining the same sizes and ratings and sometimes brand as well. Knowing how to read a tire can be some handy information to have, especially when picking out a set of new rims and you want to select the correct tires to match. Or if you are keeping your old rims and just want to widen up the contact patch of your tire you now have the information to do so.

Author: Ryan McDonald

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