Tire Size Calculator & Comparison
Compare two tire sizes (205/55R16 vs 215/45R17): diameter, speedometer diff, sidewall height, circumference and revs/mile in mm and inches. Formula shown, free and no sign-up.
* Please enter metric format (e.g., 205/55R16)
Tire Comparison Summary
Speedometer Reading vs. Actual Speed
| Indicated Speed | Actual Speed (Calculated) |
|---|
* Dimensions shown are nominal values. Actual dimensions vary by manufacturer, rim width, tire pressure, and tread wear (UN R30 6.1.4/6.1.5).
* Ride height change is a geometric theoretical value and does not account for tire deflection under vehicle load.
About & Formula
The Tire Size Calculator & Comparison Tool is a theoretical calculation simulator that determines tire dimensions—including overall diameter, section width, aspect ratio, sidewall height, circumference, and revolutions per mile/km—directly from standard metric tire codes (e.g., 205/55R16 and 215/45R17). When plus sizing or replacing wheel and tire packages, compare diameter differences, theoretical ride height variations, and speedometer discrepancies side by side.
Tire Dimension and Diameter Formulas
Dimensional calculations follow UN Regulation No. 30 Revision 3 (6.1.2.1), and inch/mile conversion factors follow the National Institute of Standards and Technology (NIST SP 811 Appendix B.8).
Calculation Examples
Single Tire Example: 205/55R16
- Section Width: 205 mm (8.07 in)
- Aspect Ratio: 55 %
- Sidewall Height: 205 × 55% = 112.75 mm (4.44 in)
- Rim Diameter: 16 × 25.4 = 406.4 mm
- Overall Diameter: 406.4 + (112.75 × 2) = 631.9 mm (24.88 in)
- Circumference: π × 631.9 = 1985.2 mm
- Revolutions: 503.7 rev/km (810.7 rev/mile)
- UN R30 Permissible New Tire Diameter Range: 625.1 to 640.9 mm
Comparison Example: 205/55R16 → 215/45R17
- New Diameter: 625.3 mm (Original: 631.9 mm)
- Diameter Difference: -6.6 mm (-1.04 %)
- Ride Height Difference: -3.30 mm
- Section Width Difference: +10 mm
- Sidewall Height Difference: -16.00 mm (96.75 mm - 112.75 mm)
- Revolutions Difference: +5.3 rev/km (+8.6 rev/mile)
- Actual Speed at 40 km/h Indicated: 39.6 km/h
- Indicated Speed at 100 km/h Actual: 101.1 km/h
References & Standards:
• Tire Dimensions & Permissible Tolerances: UN Regulation No. 30 Revision 3 (Official Journal of the European Union L 201/70)
• Inch & Mile Conversion Factors: NIST SP 811 Appendix B.8 (National Institute of Standards and Technology)
How to Use
Select Calculation Mode
Choose between 'Comparison Mode' to compare original and new tire sizes or 'Single Tire Mode' to analyze a single tire's specifications.
Enter Tire Sizes
Input metric tire sizes (e.g., 205/55R16 or 225/45R18 95W) found on your vehicle door jamb placard or tire sidewall. Quick preset buttons are also available.
Review Dimensions and Speedometer Chart
Check diameter differences, ride height change, section width, sidewall height, revolutions per mile/km, and the theoretical speedometer comparison chart.
Glossary
- Section Width
- The linear distance between the outsides of the sidewalls of an inflated pneumatic tire, excluding elevations due to labelling, decoration, or protective bands (UN Regulation No. 30, paragraph 2.12). This differs from tread contact width and varies with rim width.
- Nominal Aspect Ratio
- The ratio of section height to section width expressed as a percentage. A lower percentage indicates a shorter sidewall profile relative to tire width.
- Section Height
- The radial height from the rim seat to the outer tread surface, calculated geometrically as section width multiplied by aspect ratio divided by 100.
- Overall Diameter
- The theoretical outer diameter of an unloaded, inflated tire, calculated as rim diameter plus twice the section height.
- Circumference
- The theoretical outer circumference of the tire (π × diameter). Due to tire deflection under vehicle weight, actual dynamic rolling circumference differs from theoretical geometric circumference.
- Plus Sizing
- Increasing the wheel rim diameter while decreasing tire aspect ratio to keep overall diameter close to factory specification.
- Speedometer Variance
- Speedometer discrepancy caused by altering tire diameter. A larger tire results in a higher actual speed than displayed on the vehicle instrument cluster.
- Load Index (LI)
- An international numerical code corresponding to the maximum load a tire can support at specified inflation pressures (e.g., 91 = 615 kg, 95 = 690 kg).
- Speed Symbol
- An alphabetical symbol designating the maximum speed capability of the tire under its rated load (e.g., H = 210 km/h, V = 240 km/h, W = 270 km/h).
- Nominal Dimension
- The stated specification size on the tire sidewall. Physical manufactured dimensions vary within tolerances defined by standards such as UN Regulation No. 30 paragraph 6.1.5.
FAQ
- Q. Is my entered tire or vehicle data sent to any server?
- No. All calculations are executed entirely within your web browser using client-side JavaScript. No data is ever transmitted, tracked, or stored on external servers.
- Q. Why is it important to compare tire outer diameter when plus sizing?
- Maintaining a close overall diameter minimizes speedometer and odometer variance and helps avoid physical contact with suspension or wheel well components.
- Q. How is theoretical ride height change calculated?
- Ride height change is calculated as half of the outer diameter difference: (New Diameter - Old Diameter) ÷ 2. A positive value raises ride height, while a negative value lowers it.
- Q. Why does speedometer error occur when changing tire sizes?
- Vehicle speedometers measure wheel rotation speed. If the tire diameter increases, the vehicle travels further per revolution, making actual speed higher than the displayed speed.
- Q. What do the letters 'R' and 'ZR' mean in tire sizes?
- 'R' designates radial construction. 'ZR' indicates radial tires historically used for speeds exceeding 240 km/h (149 mph), now commonly paired with speed symbols W or Y.
- Q. What do the numbers and letters at the end of a size mean (e.g., 91V)?
- The number (e.g., 91) is the Load Index indicating maximum load capacity (91 = 615 kg). The letter (e.g., V) is the Speed Symbol indicating maximum design speed (V = 240 km/h).
- Q. Does this tool determine legal vehicle inspection pass or fail?
- No. Vehicle regulations and speedometer tolerances vary by country and region. This tool provides theoretical calculations only and does not provide legal or safety certifications.
- Q. Why do measured physical tire dimensions differ from calculation?
- Tire size codes are nominal values. According to UN Regulation No. 30, manufactured tires have permissible dimensional variations depending on tread design, rim width, inflation pressure, and wear.
- Q. Can I calculate flotation inch tires or light truck (LT) sizes?
- This tool is designed specifically for metric passenger car tires (ISO metric). Flotation sizes (e.g., 31x10.50R15) and commercial LT sizes follow different standards and are not supported.
- Q. What critical factors should I check when changing tire sizes?
- In addition to diameter and speedometer differences, verify that the new tires maintain equal or higher load index ratings and do not contact suspension or wheel well components.
Use Cases
Plus Sizing Wheel Simulation
When increasing wheel diameter from 16 inches to 17 or 18 inches, test different aspect ratios and widths before changing tires. Calculate overall diameter and ride height differences to find combinations that keep outer dimensions close to OEM.
Winter Tire Minus Sizing
When decreasing wheel diameter for winter snow tires, verify compatible tire profiles. Quickly calculate theoretical diameter differences and speedometer discrepancies to maintain similar overall profiles.
Speedometer Difference Verification
When changing tire sizes, inspect theoretical discrepancies between the speedometer reading and actual road speed. Use the 40 to 120 mph comparison table to check speed values across standard driving ranges.
Used Tire & Wheel Comparison
When considering pre-owned wheels or tires, compare candidate specifications against factory standards. Instantly check overall diameter and theoretical ride height differences directly in your browser.
Disclaimer
The calculations provided by this tool (including diameter differences, ride height variance, and speedometer differences) are theoretical geometric values based on industrial standard nominal dimensions (such as UN R30). Actual vehicle speedometer error is subject to individual factory calibration and sensor tolerances. Real tire dimensions also vary depending on brand, specific model, wheel rim width, air pressure, load weight, and tread wear. Always consult your vehicle owner's manual, tire specialist, or authorized service technician before modifying tire or wheel sizes.
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