Tire Size Calculator
Changing tire sizes can affect more than the look of your SUV. A replacement tire with a different overall diameter can alter how far the vehicle travels with each wheel revolution and may affect the speedometer reading. The tire size calculator helps you compare two metric tire sizes by estimating their nominal overall diameters and reporting the percentage difference.
Use the result as a starting point when researching replacement tires or a different wheel-and-tire combination. The estimate is based on the dimensions printed in a tire size and assumes an unloaded tire. Actual dimensions and vehicle fitment can vary.
What the Tire Size Calculator does
The Tire Size Calculator compares the estimated overall diameter of an original tire with that of a new tire. Its result, labeled Diameter Change, shows how much larger or smaller the new tire is as a percentage of the original tire’s diameter.
A positive result means the new tire has a larger calculated diameter. A negative result means it has a smaller calculated diameter. A result of 0.00% means the two sizes have the same nominal calculated diameter, although they may still differ in width, sidewall shape, construction, and fitment.
The calculator uses three measurements for each tire: width in millimeters, aspect ratio as a percentage, and wheel diameter in inches. These numbers are commonly found in a metric tire size such as 225/65R17:
- 225 is the nominal tire width in millimeters.
- 65 is the aspect ratio, meaning the nominal sidewall height is 65% of the tire’s width.
- 17 is the diameter of the wheel the tire fits, measured in inches.
The letter R indicates radial construction. It is part of the tire marking, but it is not a value you need to enter into this calculator.
How to use the Tire Size Calculator
Find the metric size printed on the sidewall of your original tire and the size of the replacement you want to compare. Then enter the numbers in the corresponding fields:
- Enter the original tire width in millimeters.
- Enter the original aspect ratio as a percentage.
- Enter the original wheel diameter in inches.
- Enter the new tire’s width, aspect ratio, and wheel diameter in the matching new tire fields.
- Read the Diameter Change result, displayed as a percentage to two decimal places.
For example, if the sidewall says 225/65R17, enter 225 for width, 65 for aspect ratio, and 17 for wheel diameter. Enter the numbers only; the field labels already identify the units.
The default original and new tire sizes are both 225/65R17, so the default result is 0.00%. Change the new tire values to compare a different size. The calculator accepts widths from 100 to 400 mm, aspect ratios from 25% to 90%, and wheel diameters from 10 to 30 inches.
How the Tire Size Calculator formula works
The calculator first estimates each tire’s sidewall height. It multiplies the tire width by the aspect ratio and divides by 100:
Sidewall height = tire width × aspect ratio ÷ 100
Next, it converts wheel diameter from inches to millimeters using 25.4 millimeters per inch. Because there is a sidewall above and below the wheel, it adds twice the sidewall height to the converted wheel diameter:
Overall diameter = wheel diameter in inches × 25.4 + 2 × sidewall height
Finally, the calculator subtracts the original diameter from the new diameter, divides the difference by the original diameter, and multiplies by 100:
Diameter Change = (new diameter − original diameter) ÷ original diameter × 100
This is a comparison of nominal dimensions, not a measurement of the loaded tire on your vehicle.
Worked example: comparing two tire sizes
Suppose the original tire is 200/50R20 and the proposed replacement is 210/50R20. Both use a 20-inch wheel and have a 50% aspect ratio, but the replacement is wider.
- Original sidewall height: 200 × 50 ÷ 100 = 100 mm.
- Original overall diameter: 20 × 25.4 + 2 × 100 = 708 mm.
- New sidewall height: 210 × 50 ÷ 100 = 105 mm.
- New overall diameter: 20 × 25.4 + 2 × 105 = 718 mm.
- Diameter Change: (718 − 708) ÷ 708 × 100 = 1.41%, rounded to two decimal places.
Although the aspect ratio stays the same, the wider tire has a taller calculated sidewall because the ratio is a percentage of width. In this example, the new tire’s nominal diameter is 10 mm larger.
Use cases for the Tire Size Calculator
Comparing replacement options: If you are considering a different metric size for your SUV, the calculator gives you a quick way to see how its nominal diameter compares with the original. This can help you narrow down options before checking fitment details.
Evaluating a wheel change: A larger wheel does not automatically mean a larger overall tire diameter. A tire with a lower aspect ratio may offset some or all of the increase in wheel diameter. Enter the complete original and proposed sizes to see their combined effect.
Understanding sidewall changes: Two tires with the same aspect ratio can have different sidewall heights if their widths differ. Comparing the full size helps you understand why a wider or narrower tire may change the overall diameter.
Discussing options with a tire professional: A percentage comparison gives you a useful starting point for a conversation about replacement tires. Bring the original size, the proposed size, and your vehicle information so the professional can assess factors the calculator does not cover.
Other factors to consider when calculating Diameter Change
Actual tire dimensions can differ. The numbers on a tire describe nominal dimensions. Tire model, inflation pressure, load, and wear can affect actual dimensions and the way a tire sits on the road. A calculated match is therefore not a guarantee that two tires will measure identically in use.
Diameter alone does not establish fitment. A tire also needs adequate clearance around the body, suspension, brakes, and steering components throughout the vehicle’s range of movement. Tire width and wheel specifications matter as well. A small diameter change does not prove that a tire will fit safely.
Check load and speed suitability. The replacement tire must be appropriate for the vehicle and its intended use. Review the vehicle manufacturer’s requirements and the tire’s load and speed markings; those specifications are outside this calculator’s result.
Consider speedometer effects. If the effective rolling circumference changes, the distance traveled per wheel revolution changes too. With a larger effective circumference, actual speed may be higher than the indicated speed; with a smaller one, it may be lower. The calculator compares nominal diameter only, so it does not predict an exact speedometer reading.
Follow vehicle-specific guidance. Consult your owner’s manual, the vehicle’s tire information label, and a qualified tire professional when choosing a replacement. This is especially useful when changing wheel sizes or selecting tires for an all-wheel-drive SUV, where the manufacturer may specify requirements for matching tires.
Tire Size Calculator FAQ
What does a negative Diameter Change mean?
A negative percentage means the new tire’s calculated nominal overall diameter is smaller than the original tire’s diameter. For example, a result of −2.00% means the new calculated diameter is 2% smaller, using the original diameter as the reference.
Does 0.00% mean the tires are interchangeable?
No. It means the entered sizes produce the same nominal diameter under the formula. Width, wheel compatibility, clearance, load rating, speed rating, and other vehicle-specific requirements still need to be checked.
Can I compare tires for different wheel diameters?
Yes. Enter each tire’s corresponding wheel diameter in inches along with its width and aspect ratio. The calculation includes the wheel diameter and both sidewalls when estimating overall diameter.
Why does changing tire width affect diameter?
The aspect ratio expresses sidewall height as a percentage of tire width. If the aspect ratio stays the same, a wider tire has a taller nominal sidewall, which increases its calculated overall diameter.
Does the calculator measure the tire on my SUV?
No. It estimates unloaded nominal diameter from the size values you enter. Actual dimensions and vehicle fitment can vary, so check the tire manufacturer’s specifications and vehicle requirements before making a purchase.