Tire Size Calculator
Use this tire size calculator to compare stock and replacement tire dimensions side by side — overall diameter, sidewall height, circumference, radius change, revolutions per mile, and estimated speedometer calibration variance.
Compare Tire Sizes
Stock vs ReplacementVisual Tire Comparison
Compare stock and replacement tire dimensions to scale.
Detailed Geometry ComparisonStock vs Replacement
| Metric Parameter | Stock (225/45R17) | Replacement (245/45R17) | Difference |
|---|---|---|---|
| Section Width | 225 mm (8.86″) | 245 mm (9.65″) | +20 mm |
| Sidewall Height | 101.3 mm (3.99″) | 110.3 mm (4.34″) | +9.0 mm |
| Overall Diameter | 24.97″ (634.3 mm) | 25.68″ (652.3 mm) | +0.71″ |
| Circumference | 78.45″ (1993 mm) | 80.68″ (2049 mm) | +2.23″ |
| Revolutions per Mile | 808 revs/mi | 785 revs/mi | -23 revs/mi |
| Revolutions per Km | 502 revs/km | 488 revs/km | -14 revs/km |
Speedometer Calibration Variance
Calculate actual driving speed at custom indicated speeds.
Calculations use documented geometric and mathematical relationships, with tire-industry standards and manufacturer specifications referenced where relevant.
How to Use the Tire Size Calculator
Enter stock and replacement tire sizes to compare overall diameter, geometric radius change, revolutions per mile, and estimated speedometer differences.
- Enter your original factory tire specification into the Stock Tire field using direct text entry (e.g.
225/45R17or flotation size33x12.50R15) or dropdown selectors. - Enter your replacement size into the Replacement Tire field.
- Review the Diameter Difference %, Diameter Change, and Radius / Clearance Change result cards.
- Use the interactive Visual Tire Comparison to inspect tire cross-section profiles side by side or overlaid.
Worked Example: 225/45R17 vs 245/45R17
The following tire size comparison table shows the calculator results for a factory 225/45R17 tire compared against a wider 245/45R17 replacement size:
| Measurement Parameter | Stock (225/45R17) | Replacement (245/45R17) | Calculated Difference |
|---|---|---|---|
| Section Width | 225 mm (8.86″) | 245 mm (9.65″) | +20 mm (+0.79″) |
| Sidewall Height | 101.3 mm (3.99″) | 110.3 mm (4.34″) | +9.0 mm (+0.35″) |
| Overall Diameter | 24.97″ (634.3 mm) | 25.68″ (652.3 mm) | +0.71″ (+18.0 mm) |
| Diameter Difference % | Baseline (0.0%) | +2.8% Variance | +2.8% |
| Radius / Clearance Change | Baseline (0.00″) | +0.35″ (+9.0 mm) | +0.35″ (+9.0 mm) |
| Circumference | 78.45″ (1993 mm) | 80.68″ (2049 mm) | +2.23″ (+56 mm) |
| Estimated Geometric Revs/Mile | 808 revs/mi | 785 revs/mi | -23 revs/mi |
Speedometer Calibration Effect: At an indicated dashboard speed of 60 mph, actual speed is approximately 61.7 mph (actual speed is approximately 2.8% higher than the indicated speed). This occurs because the taller 25.68″ replacement tire has a nominal geometric circumference of 80.68 inches, traveling 2.23 inches farther per wheel revolution than the 24.97″ stock tire.
How to Read Your Tire Comparison Results
Evaluating tire comparison outputs requires understanding both the mathematical meaning of each value and its practical fitment implications:
| Result Output | What It Means & Practical Guidance |
|---|---|
| Diameter Difference % | A ±3% diameter difference is commonly used as a quick comparison reference, but it is not a universal fitment, drivetrain, or safety limit. |
| Diameter Change | Represents the total absolute change in outer tire diameter. Positive values indicate a taller tire; negative values indicate a shorter tire. |
| Radius / Clearance Change | Tire radius equals half the overall tire diameter. This output shows the geometric radius change — half of the overall diameter change. It may approximate the vertical clearance change at components that directly follow tire radius, but actual vehicle clearance changes also depend on suspension geometry, wheel offset, wheel width, tire construction, and steering movement. |
| Section Width Difference | Indicates the nominal section width change. Actual installed fitment also depends on wheel width, wheel offset (ET), tire construction, suspension geometry, and fender clearance. |
| Revolutions per Mile | Helps estimate changes in distance per revolution, odometer behavior, and effective drivetrain gearing. These are nominal geometric estimates; real manufacturer-published rolling revolutions per mile may differ. For a dedicated revolutions per mile tool, see the Tire Revolutions Per Mile Calculator. |
| Speedometer Variance | Estimates the percentage deviation between indicated dashboard speed and actual road speed caused by changes in distance traveled per wheel revolution. |
How Overall Tire Size Geometry Is Calculated
Tire height is an informal term for overall tire outer diameter — the full outer measurement across the tire from tread surface to opposite tread surface through the center axis. Tire radius is exactly half the overall diameter. Sidewall height (the structural portion between the bead seat and tread) is calculated from the section width and aspect ratio; aspect ratio is also informally called the tire profile or sidewall profile number.
Tire specifications convey section width in millimeters, sidewall height as a percentage ratio, and wheel rim diameter in inches. This calculation engine uses standard dimensional conversion equations (1 inch = 25.4 mm, 1 mile = 63,360 inches):
Sidewall Height (mm) = Section Width (mm) × (Aspect Ratio / 100)Overall Diameter (in) = Rim Diameter (in) + 2 × [Sidewall Height (mm) / 25.4]Tire Radius (in) = Overall Diameter (in) / 2Circumference (in) = Overall Diameter (in) × πRevs/Mile = 63,360 inches / Circumference (in)Diameter Difference % = [(New Diameter - Stock Diameter) / Stock Diameter] × 100Radius Change (in) = (New Diameter - Stock Diameter) / 2Understanding Tire Size Markings: 225/45R17
A tire size such as 225/45R17 encodes nominal dimensional and construction information. U.S. regulations also require specified safety and identification information to appear on tire sidewalls:
- 225 (Nominal Section Width): The nominal tire section width in millimeters. Actual mounted section width can vary slightly by tire model, measuring rim width, and manufacturer specifications.
- 45 (Aspect Ratio): Sidewall height expressed as a percentage of nominal section width (225 × 0.45 = 101.25 mm). The aspect ratio is also informally called the tire profile or sidewall profile number — a lower number means a shorter, lower-profile sidewall.
- R (Internal Construction): Radial construction.
- 17 (Wheel / Rim Diameter): The nominal wheel diameter in inches measured at the tire bead-seat diameter, not the visible outer wheel lip.
For a complete explanation of tire markings, load index, speed rating, and sidewall codes, see How to Read Tire Size. To convert a single tire specification between metric notation and inch flotation format, use the Tire Size Conversion Calculator.
Tire Sidewall Markings & Dimensional Geometry Breakdown
How Tire Size Affects Speedometer Readings & Drivetrains
Vehicle speed calculations depend on rotational speed. Changing tire circumference changes the distance traveled per wheel revolution, which can change the relationship between indicated and actual road speed:
Taller Replacement Tire
A larger circumference travels farther per revolution, so actual road speed may be higher than the indicated speed.
Shorter Replacement Tire
A smaller circumference travels less distance per revolution, so actual road speed may be lower than the indicated speed.
AWD & 4WD Systems: All-wheel-drive and four-wheel-drive systems are often sensitive to differences in rolling circumference between individual tires. Because tolerance thresholds vary significantly by vehicle design and transfer case type, always consult the specific vehicle manufacturer's owner manual for tire size, circumference, tread-depth variance, and inflation pressure specifications.
Tire Diameter Difference, Plus Sizing & Fitment
Plus sizing pairs a larger-diameter wheel with a lower-profile tire so overall tire diameter remains reasonably close to the original size. When selecting replacement tires, follow the vehicle and tire manufacturers' recommendations. The U.S. Tire Manufacturers Association (USTMA) also publishes tire replacement and fitment guidance for passenger and light-truck tires.
Overall diameter is only one component of vehicle fitment. Fitment also depends on:
- Section Width & Rim Width: Ensuring the tire section width is mounted within the manufacturer-approved rim-width range.
- Wheel Offset (ET) & Backspacing: Ensuring replacement wheels maintain adequate inner suspension clearance and outer fender clearance.
- Load Index: Verify that the replacement tire meets the vehicle's required load capacity.
- Speed Rating: Verify the appropriate speed rating for the vehicle and intended application.
Plus-Sizing Progression: Stock (17″) vs Plus 1 (18″) vs Plus 2 (19″)
Calculation Methodology & Real-World Limitations
TireCalculator.net calculates nominal tire geometry from the dimensions encoded in the tire size designation using standard unit conversions. In the North American market, The Tire and Rim Association (TRA) publishes standardized tire designations, dimensions, load ratings, and approved rim information.
Actual mounted dimensions and real rolling behavior may vary because of tire model, measuring rim width, inflation pressure, vehicle load, casing construction, tread depth, deformation under load, and manufacturing tolerances.
Nominal section width, loaded radius, rolling circumference, and revolutions per mile can differ from geometric calculations. For close-fitment, drivetrain-sensitive, or purchasing decisions, verify the tire manufacturer's published specifications, approved rim-width range, and the vehicle manufacturer's requirements.
Frequently Asked Questions
References
- National Highway Traffic Safety Administration (NHTSA): Tire Safety, Ratings, and Regulatory Sidewall Labeling Overview. https://www.nhtsa.gov/equipment/tires
- U.S. Tire Manufacturers Association (USTMA): Replacement Guidance for Passenger and Light Truck Tires. https://www.ustires.org/replacement-tires
- The Tire and Rim Association, Inc. (TRA): Standards Organization for Tire, Rim, and Interchangeability Information. https://www.us-tra.org/