Tire Size Calculator & Visualizer (2024)

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This tool combines features of a tire size calculator with an easy way to compare speedometer differences for tire sizes. Just enter the original tire size you see on the sidewall and compare it to the desired tire size alternative.

So what to the number below stand for?

The first number (265) is the tire width in millimeters.

The 2nd number 60 is used to calculate the sidewall height or "aspect ratio", by percentage. So, the sidewall height of a 320/60 R 18 tire is 60% of 320 = 192

The 3rd number 18 (generally preceded by an "R") is Rim Diameter in inches.

This is the diameter of the rims bead seat base, not the diameter of the outer rim edge. The rim edge, or lip, extends past the tire bead seat to hold the bead and stop the tire slipping off the rim.

When we say a rim has a 18 inch diameter, we mean that the diameter of the bead seat base measures 18 inches. The outer rim edge diameter will actually measure more, depending on the design.

Odometer Reading

If you replace 320/60/18 with 235/60/18 for Odometer reading 1000 the actual distance traveled = 879 ~ (-12.1%)

(If your Odometer is accurate for original tires)

Edit and Copy Tire diagram above to compare or print.

Speedo Difference between Wheels 12.1%

320/60 R 18 KPH 235/60 R 18 KPH
5 4.4
10 8.8
15 13.2
20 17.6
25 22
30 26.4
35 30.8
40 35.1
45 39.5
50 43.9
55 48.3
60 52.7
65 57.1
70 61.5
75 65.9
80 70.3
85 74.7
90 79.1
95 83.5
100 87.9
105 92.3
110 96.7
115 101.1
120 105.4
125 109.8
130 114.2
135 118.6
140 123
145 127.4
150 131.8

How to use our tire size calculator?

To get an accurate difference between your original tires and alternative size tires you should do this:

  • Go to the “Tire 1” and enter the tire width in millimeters, the aspect ratio ( the ratio of tire height to tire width), and the rim diameter of your current tires. Note that they must be within recommended limits or otherwise, the calculator will not give you an accurate result.
  • Proceed to the “Tire 2” and fill in the corresponding fields for your alternative size tires.
  • Press “Calculate”.

This tool is here to provide you with the necessary information, but it must be used with caution

  • When using the tire circumference calculator, please remember that the model and brand of the respective car are not considered.
  • Due to the size of the brake discs, it is impossible to replace the 225/40 R 18 tires with the 185/80 R13 tires mounted on 18-inch rims.
  • Conversely, the 185/80 R13 models can't replace 225/40 R18 tires.

You should generally seek professional advice when looking for an alternative tire.

Tire Size Calculator

One day you may want to use aftermarket tires different from the factory rubber supplied with your vehicle. Alternative sizes for a tire make sense if you want to influence unique driving characteristics, such as:

  • braking distance
  • ground clearance
  • off-roading modifications
  • improving torque
  • more contact with the road surface with wider wheels
  • better cornering thanks to wider thread and different sidewalls
  • by going for bigger or smaller tires, you can change the appearance of your car

Prices for alternative tires can differ significantly from the original tires - sometimes, they are cheaper or more expensive.

What tire information do you need?

The dimensions of a tire are usually specified in this way: 175/75 R14.

  • The first value (175) indicates the width of the unloaded tire in millimeters.
  • The second value (75) indicates the ratio of the sidewall height to tire width in percent. If this value is missing, then it is 80.
  • The sidewall height is the tire thickness, the distance between the rim's outer edge and the tire's outer edge.
  • R means radial tires, the most common form of tires today.
  • The third value (14) indicates the diameter of the rim in inches [b]. An inch is 2.54 centimeters.

What is the speed index?

This is the maximum speed at which you can drive the tires. If this is exceeded, the tires are dangerous to use. The speed index is indicated by letters, e.g. T (190 km/h) V (240 km/h). You should permanently mount the tires with the speed index approved by the manufacturer.

What is the load index?

This is denoted by a LI symbol (Load Index). This describes the maximum load capacity of the tire in kilograms. The load index is represented by numbers, e.g., 91 means that the full load on the tire is 615 kg, and this cannot be exceeded. You should always fit tires with the load index recommended by the manufacturer.

Approved tire size

Determining an alternative tire options only gives a meaningful result in a direct comparison with the size approved for your car. If your car already has tires that differ in size from the OEM tires, then the alternative determined by the tire calculator is by no means suitable.

The numbers will not show the true picture. The alternative calculation only gives a correct result compared to the size approved for your vehicle. So always compare the new tires' size to the recommended value. If the tires fitted to your car are already alternatives, the alternative calculated with them may not fit your vehicle.

You can find the permitted tire size for your car in the owner's manual or with the help of our tire calculator.

Important notes:

With a diameter/circumference and speed calculator, you can quickly and easily test whether a speedometer adjustment is necessary for your desired tire size.

When changing the ground clearance, some rounding errors are possible, but they amount to a maximum of 1 mm, i.e., and are irrelevant.

The speedometer may show a maximum of 8% too much and no km/h too little. Please note that the speedometer display is slightly ahead of the factory original on most vehicles.

How much difference in tire diameter is acceptable?

If you want to go through the tire conversion process to improve your driving experience, you should know the safe limits. By installing the alternative-size tires, you change not only the way your car behaves but also the way other vital systems function after this sudden change. So to stay within the safe threshold, keep the difference at 3% of the original diameter/height. This cautious approach will save you from faulty brakes and other problems.

Is it OK to change the tire aspect ratio?

The tire aspect ratio is, in simple words, the relationship between the width and height of the tire.

The aspect ratio is the width of the tire divided by its height. The higher/taller the sidewall (or “profile”), the bigger the aspect ratio. The wider and shorter the tire, the lower the aspect ratio.

Car owners often resolve to lower aspect ratio to get better performance and improved control over the vehicle. But that must be done with extra caution and ideally with help from a professional.

Because if you mount a lower aspect ratio tire on the same diameter wheels, you'll need to increase the tread width to keep the speedometer and ECU readings consistent. And compensation can be tricky sometimes.

How do you convert tire size to height?

To calculate the sidewall height, multiply the ratio (70 or 0.70) by the width (225): 225 millimeters x 0.70 = 157.5 millimeters. The number 15, or R15, is the diameter of the wheel in inches.

How do I know my tire size is compatible?

The most reliable source of information is your car's manual, it contains all the necessary information. You can also use the recommended tire size values from the manual to test your chosen alternatives using our tire size comparison calculator to get a better idea.

Also, by changing one size up in tire width and one size down in aspect ratio (the proportion of the tire's height to width), you will stay within a reasonable range.


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