Tire Size Calculator
This tool calculates and compares the geometric dimensions of automotive tires based on their ISO metric codes. By inputting the specifications of your current tire and a proposed replacement, you can quantify changes in overall diameter, sidewall height, and circumference.
Modifying tire sizes affects vehicle dynamics, including speedometer calibration, ground clearance, and effective gear ratio. This calculator applies standard engineering formulas to determine the percentage difference between two tire sizes, helping to identify fitments that remain within manufacturer safety tolerances.
Governance
Record 668ec3a801fb • Reviewed by Fidamen Standards Committee
Compare two standard metric tire sizes (e.g., 205/55R16).
Inputs
Advanced inputs
Current Tire Size
New Tire Size
Results
Stock Diameter
12.126
New Diameter
12.126
Difference
0.00%
True Speed @ 65mph
MPH 65.00
| Output | Value | Unit |
|---|---|---|
| Stock Diameter | 12.126 | in |
| New Diameter | 12.126 | in |
| Difference | 0.00% | — |
| True Speed @ 65mph | MPH 65.00 | mph |
Visualization
Methodology
The calculator derives tire dimensions using the ISO 4000-1 standard naming convention (e.g., 225/45R17). The section width is converted from millimeters to inches. The sidewall height is calculated as the product of the section width and the aspect ratio (percentage).
Total tire diameter is the sum of the rim diameter and twice the calculated sidewall height. Circumference is the product of the diameter and Pi (π). Revolutions per mile are derived by dividing 63,360 (inches in a mile) by the tire circumference.
Speedometer error is calculated by comparing the rotational distance of the new tire against the original equipment (stock) tire. If the new tire has a larger circumference, it travels further per revolution, causing the speedometer to read lower than the vehicle's actual ground speed.
F.A.Q.
What is the 3% rule for tire sizing?
Automotive engineers and safety experts generally recommend keeping the total diameter of replacement tires within 3% of the original equipment (OEM) specification. Exceeding this limit can adversely affect anti-lock braking systems (ABS), traction control, and transmission shift points.
How does tire size affect speedometer accuracy?
Speedometers are calibrated to the circumference of the factory-installed tires. A larger tire rotates fewer times to cover the same distance, causing the speedometer to display a speed lower than your actual velocity. Conversely, a smaller tire will cause the speedometer to read higher than your true speed.
What do the numbers on a tire sidewall mean?
The first number (e.g., 205) is the width of the tire in millimeters. The second number (e.g., 55) is the aspect ratio, indicating the sidewall height is 55% of the width. The third number (e.g., 16) is the diameter of the wheel (rim) in inches.
Sources & citations
- NHTSA - Tires & Safety — https://www.nhtsa.gov/equipment/tires
- ISO 4000-1:2021 Passenger car tyres and rims — https://www.iso.org/standard/79701.html
- Tire and Rim Association (TRA) Standards — https://www.us-tra.org/
- EPA — FuelEconomy.gov Official Fuel Economy Information — https://www.fueleconomy.gov/
- EPA — 40 CFR Part 600 Fuel Economy Regulations — https://www.ecfr.gov/current/title-40/chapter-I/subchapter-Q/part-600
- SAE International — Automotive Engineering Standards — https://www.sae.org/
Further resources
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
Record ID: 668ec3a801fbWhat changed (latest)
v1.0.0 • 2025-11-30 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Versioning & Change Control
Audit record (versions, QA runs, reviewer sign-off, and evidence).
What changed (latest)
v1.0.0 • 2025-11-30 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Public QA status
PASS — golden 25 + edge 120
Last run: 2026-01-23 • Run: golden-edge-2026-01-23
Engine
v1.0.0
Data
Baseline (no external datasets)
Content
v1.0.0
UI
v1.0.0
Governance
Last updated: Nov 30, 2025
Reviewed by: Fidamen Standards Committee (Review board)
Credentials: Internal QA
Risk level: low
Reviewer profile (entity)
Fidamen Standards Committee
Review board
Internal QA
Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person
Semantic versioning
- MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
- MINOR: New features or fields that do not change existing outputs for the same inputs.
- PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.
Review protocol
- Verify formulas and unit definitions against primary standards or datasets.
- Run golden-case regression suite and edge-case suite.
- Record reviewer sign-off with credentials and scope.
- Document assumptions, limitations, and jurisdiction applicability.
Assumptions & limitations
- Uses exact unit definitions from the Fidamen conversion library.
- Internal calculations use double precision; display rounding follows the unit's configured decimal places.
- Not a substitute for calibrated instruments in regulated contexts.
- Jurisdiction-specific rules may require official guidance.
Change log
v1.0.0 • 2025-11-30 • MINOR
Initial publication and governance baseline.
Why: Published with reviewed formulas, unit definitions, and UX controls.
Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 536265926f05