Tire Size Calculator
Compare two tire sizes for overall diameter, section width, circumference, and speedometer error. Supports 225/45R17 metric and 31x10.50R15 flotation codes.
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What is Tire Size Calculator?
The Tire Size Calculator is a free online tool that compares two sidewall size codes and reports overall diameter, section width, sidewall height, circumference, revolutions, and speedometer error. It understands passenger metric sizes such as 225/45R17 and flotation sizes such as 31x10.50R15, all in your browser.
Key Benefits
- Know diameter and width change before ordering a plus-size or replacement tire
- Estimate true speed versus the indicated speedometer after a size swap
- Catch replacements that drift outside the common 3% diameter guideline
- Works offline with no VIN lookup or dealer account
Common Use Cases
- •Plus-sizing 225/45R17 to 245/40R18 and checking speedometer impact
- •Comparing a truck flotation size with a metric equivalent
- •Verifying that a cheaper replacement stays close in overall diameter
- •Sharing a copyable diameter and speed summary with a tire shop
How to Use the Tire Size Calculator
- Enter the original size: Type the size stamped on your current tire, such as 225/45R17 or 31x10.50R15. A load/speed index like 91V is ignored.
- Enter the new size: Type the replacement or plus-size you are considering, for example 245/40R18.
- Read diameter and width deltas: Check overall diameter, section width, and the 3% diameter guideline. Staying close in diameter keeps speedometer error and gearing closer to stock.
- Check actual speed: Use the speedometer table to see true speed at 60–120 km/h indicated, then copy the comparison summary if you want to save it.
Key Features
- Parses metric (225/45R17) and flotation (31x10.50R15) sidewall codes
- Shows overall diameter, width, sidewall height, and circumference in mm and inches
- Estimates speedometer error after a size change at common indicated speeds
- Flags replacements that exceed the common 3% overall-diameter guideline
- Accepts P/LT prefixes, ZR construction, and trailing load/speed indexes
- Runs entirely in-browser — copy a summary, nothing is uploaded
Frequently Asked Questions
How is overall tire diameter calculated?
For a metric size WWW/AA R DD, sidewall height is width × aspect ratio, and overall diameter is two sidewalls plus the rim diameter in millimeters (rim inches × 25.4). Flotation sizes like 31x10.50R15 already state overall diameter in inches.
Why does a different tire size change the speedometer?
The speedometer counts wheel revolutions. A taller tire covers more ground per revolution, so the indicated speed is lower than true speed. Actual speed ≈ indicated × (new diameter ÷ original diameter).
What does the 3% diameter rule mean?
Many tire shops recommend keeping overall diameter within about 3% of the original so ABS, speedometer, and fender clearance stay close to spec. This tool flags larger changes; always confirm with your vehicle maker and a fitter.
Does this tool work offline?
Yes. Parsing and comparison run in your browser. No tire sizes or vehicle data are uploaded.