Tyre size comparison
Enter the original and proposed metric radial sizes. This geometric comparison does not establish whether a tyre is approved for your vehicle.
Enter your values, then calculate.
Method and limitations
NHTSA explains metric tyre markings: in 205/55 R16, 205 is nominal width in mm, 55 is sidewall height as a percentage of width, R denotes radial construction and 16 is rim diameter in inches. Sidewall = width × ratio / 100; diameter = rim × 25.4 + 2 × sidewall; circumference = π × diameter. Change = (new diameter / original diameter − 1) × 100. Estimated actual speed = indicated speed × new / original diameter, assuming the speedometer was accurate with the original tyres. Actual minus indicated is shown in your selected speed unit. These are nominal, unloaded dimensions; pressure, wear and construction affect rolling circumference. This does not check rim fit, clearance, load or speed ratings, approval or existing speedometer error. NHTSA recommends the original size or a size recommended by the vehicle manufacturer; no universal percentage tolerance is applied. NIST: 1 inch = 25.4 mm exactly. NIST: https://www.nist.gov/system/files/documents/2023/01/30/appc-23-HB44.pdf ; NHTSA: https://www.nhtsa.gov/document/tire-every-thing-rides-it-brochure ; https://www.nhtsa.gov/vehicle-safety/tires
Sources
- NIST Handbook 44 (2023), Appendix C — National Institute of Standards and Technology,
- Tire Safety: Everything Rides on It — NHTSA,
- Tires — Tire Size — NHTSA,