About TyreCalculator Accurate tire data backed by industry standards
What We Do
TyreCalculator is a free reference tool for drivers, mechanics, and tire professionals. We help you compare tire sizes, check speedometer accuracy, find alternative fitments, and look up factory tire specifications for popular vehicles.
Every calculation on this site follows the standard tire sizing formulas defined by the U.S. Tire Manufacturers Association (USTMA) and the European Tyre and Rim Technical Organisation (ETRTO).
Our Data Sources
We compile tire and vehicle data from multiple authoritative sources to ensure accuracy:
- Vehicle specifications — sourced from manufacturer spec sheets, owner's manuals, and the tire placards required by NHTSA (National Highway Traffic Safety Administration)
- Tire dimensions — calculated using ETRTO/USTMA standard formulas from the nominal size designation (width, aspect ratio, rim diameter)
- Speed ratings and load indices — referenced from the ETRTO Standards Manual and cross-checked against manufacturer data
- Bolt patterns and offsets — compiled from OEM service manuals and verified against aftermarket fitment databases
How Our Calculations Work
All tire size calculations use the internationally recognized metric tire sizing system. For a tire marked as 225/65R17:
Sidewall Height = 225 × 0.65 = 146.25 mm
Overall Diameter = (146.25 × 2) + (17 × 25.4) = 724.3 mm
Circumference = 724.3 × π = 2,275 mm
Revolutions per Mile = 1,609,344 / 2,275 = 707.6
Speedometer correction is calculated by comparing the circumference of two tire sizes. The industry-accepted tolerance for replacement tires is within 3% of the original overall diameter, as recommended by most vehicle manufacturers and referenced in ETRTO guidelines.
Safety Information
The data on this site is provided for informational and educational purposes. Tire selection affects vehicle safety, handling, fuel economy, and warranty coverage. We strongly recommend:
- Consulting your vehicle's owner's manual or tire placard for OEM specifications
- Having a qualified tire professional verify any alternative size before installation
- Checking local regulations — some jurisdictions restrict tire modifications that alter speedometer accuracy beyond legal limits
- Never exceeding the load or speed rating required for your vehicle
For detailed tire safety information, visit the NHTSA Tire Safety page or the Tire Rack educational resources.
Contact
Found an error in our data? Have a suggestion? Email us at [email protected]. We review all corrections and update our database regularly.