The Tire Size Calculator computes tire dimensions from the size code (e.g., 195/65R15).
Displays sidewall height, total diameter, circumference, and revolutions per km. Comparison mode compares two tire sizes with speedometer correction and ground clearance changes.
Calculator information
๐ How to use this calculator
- Enter the tire code in Width/Aspect R Diameter format (for example 215/65R17).
- Confirm that width is in millimeters and rim diameter is in inches - this is the global standard.
- View the calculated sidewall height, overall tire diameter, circumference, and revolutions per mile.
- For comparison mode, enter a second tire size to compare dimensions side by side.
- Watch the diameter deviation; differences above 3 percent can affect speedometer accuracy.
- Review the change in ground clearance and the estimated speedometer correction when switching tire sizes.
๐งฎ Tire Code to Metric Dimensions
Diameter = (Rim x 25.4) + 2 x (Width x Aspect/100); Circumference = pi x Diameter
- Width: tire width in mm (for example 215)
- Aspect: aspect ratio as a percentage (for example 65)
- Rim: rim diameter in inches (for example 17)
- 25.4: conversion factor from inches to mm
- Sidewall = Width x Aspect / 100
Overall diameter deviation should ideally stay within 3 percent of the OEM size to avoid disrupting ABS, traction control, and speedometer readings.
๐ก Worked example: Tire 215/65R17 (common on Toyota RAV4, Honda CR-V)
Given:- Width: 215 mm
- Aspect ratio: 65%
- Rim diameter: 17 inches
Steps:- Sidewall: 215 x 65 / 100 = 139.75 mm
- Rim diameter in mm: 17 x 25.4 = 431.8 mm
- Overall diameter: 431.8 + 2 x 139.75 = 711.3 mm (about 28.0 inches)
- Circumference: pi x 711.3 = 2234.4 mm, or about 7.33 ft
- Revolutions per mile: 5280 / 7.33 = approximately 720 revolutions
Result: A 215/65R17 tire has an overall diameter of 28.0 inches, a circumference of 7.33 ft, and rotates about 720 times per mile.
โ Frequently asked questions
What do the numbers in 215/65R17 mean?
215 is the tread width in millimeters. 65 is the sidewall height as a percentage of the width (65% of 215 mm = 139.75 mm). R indicates radial construction. 17 is the rim diameter in inches. After that you usually see the load index and speed rating, for example 99H meaning a maximum load of 1,709 lb and a maximum speed of 130 mph.
What is the safe tolerance when changing tire sizes?
Keep total diameter deviation within 3 percent of the OEM size. Beyond that, the speedometer becomes inaccurate, ABS and stability control can be affected, automatic transmission shift points may shift, and ride-height sensors can misread. When plus-sizing (going up 1 or 2 inches on the wheel), pick a lower aspect ratio so the overall diameter stays close to stock.
Are wider tires always better?
Not automatically. Wider tires increase dry grip and improve looks but they reduce fuel economy, add steering effort, and can worsen wet-weather performance because they are more prone to hydroplaning. They also add load on the suspension and bushings. Stick to the manufacturer's recommended width unless you understand the trade-offs.
Why does the speedometer read faster than my actual speed?
Under FMVSS 101 and the broadly adopted UN ECE R39 standard, a speedometer must read at or above the true speed, never below it. Typical tolerance is the indicated speed minus the true speed being between 0 and roughly 10 percent plus 4 km/h (about 2.5 mph). Smaller-than-stock tires worsen this over-read; larger tires can make the speedometer read low, which is illegal in many jurisdictions.
How do I read the tire manufacture date?
Look for the 4-digit DOT code on the sidewall, usually inside an oval. The first two digits are the week of production and the last two are the year. For example, 2624 means the 26th week of 2024. NHTSA and most manufacturers recommend replacing tires that are 6 to 10 years old regardless of tread depth, because the rubber hardens with age.
๐ Sources & references
Last updated: May 11, 2026