Research tire sizes, understand what the numbers on a tire mean, compare alternative sizes, and learn which measurements matter when choosing tires for your vehicle.
FitmentCompass combines tire size explanations, calculators, vehicle fitment resources, and practical guides so you can understand both the numbers and how they may affect your vehicle.
Understanding Tire Sizes
Most passenger-vehicle tire sizes use a format such as 225/60R17. Each part of the size describes a different characteristic of the tire. For a complete explanation of tire markings, see our How to Read a Tire Size guide.
Example: 225/60R17
| Marking | Meaning | What It Tells You |
|---|---|---|
| 225 | Tire width | Nominal section width of approximately 225 mm. |
| 60 | Aspect ratio | The sidewall height is approximately 60% of the tire width. |
| R | Construction | Indicates radial tire construction. |
| 17 | Wheel diameter | The tire is designed for a 17-inch wheel. |
Compare Tire Sizes
Changing tire size can affect overall diameter, width, sidewall height, circumference, ride height, revolutions per mile, and speedometer readings.
Use our Tire Size Comparison Calculator to compare two sizes side by side before researching whether the alternative size is suitable for your vehicle.
Tire Size Comparison Calculator
Compare width, sidewall height, overall diameter, circumference, revolutions per mile, ride height, and estimated speedometer differences between two tire sizes.
Tire Size Calculators and Tools
Use these tools when you need to calculate tire dimensions, convert tire sizes, check diameter, or estimate speedometer changes.
Tire Size Calculator
Calculate width, sidewall height, overall diameter, circumference, and other measurements from a tire size.
Open Calculator →Tire Size Converter
Convert and compare common tire sizing measurements and better understand equivalent dimensions.
Open Converter →Tire Diameter Calculator
Calculate approximate overall tire diameter using width, aspect ratio, and wheel diameter.
Calculate Diameter →Speedometer Difference Calculator
Estimate how changing overall tire diameter may affect indicated and actual vehicle speed.
Check Speedometer Difference →Key Tire Size Measurements
Tire Width
Tire width is usually shown in millimeters. A wider tire may change inner suspension clearance and outer fender clearance even when overall diameter remains similar.
Aspect Ratio
The aspect ratio describes sidewall height as a percentage of tire width. A 60-series tire has a taller sidewall relative to its width than a 45-series tire.
Wheel Diameter
The final number in a metric tire size indicates the wheel diameter the tire is designed to fit. A 17-inch tire must be mounted on a compatible 17-inch wheel.
Why Overall Tire Diameter Matters
Overall diameter affects how far the vehicle travels during each tire revolution. Changing it can influence speedometer readings, effective gearing, ride height, wheel-well clearance, and revolutions per mile. Use the Tire Diameter Calculator when you need to calculate overall diameter from a tire size.
A commonly referenced starting guideline is to keep an alternative tire’s overall diameter relatively close to the original size, often within approximately 3%. That percentage can be useful for comparison, but it does not guarantee vehicle fitment.
If the new tire has a different overall diameter, our Speedometer Difference Calculator can help estimate how the change may affect indicated vehicle speed.
Choosing an Alternative Tire Size
An alternative tire size should be evaluated using more than diameter alone. Two sizes can have nearly identical overall diameters while having substantially different widths or requiring different wheel dimensions. See our Can You Use a Different Tire Size? guide for a detailed explanation.
Before changing tire size, consider:
- Overall tire diameter and circumference
- Tire width and available clearance
- Wheel width compatibility
- Wheel diameter
- Load index and load capacity
- Speed rating
- Recommended tire pressure
- Brake and suspension clearance
- Vehicle manufacturer specifications
- Any suspension or body modifications
Load Index and Speed Rating
A complete tire specification often includes additional markings after the tire size, such as 99H. These markings describe the tire’s load index and speed rating.
Choosing a tire with similar dimensions does not automatically mean it has the correct load capacity or speed rating for the vehicle. Always verify these specifications before replacing the original tires.
Factory Tire Size vs. Alternative Tire Size
The factory tire size is selected as part of the vehicle’s original engineering and may vary by trim, wheel package, drivetrain, market, or model year.
Some vehicles are offered with several factory-approved tire sizes. Other sizes may also physically fit, but compatibility should be verified rather than assumed.
When researching your exact vehicle, use our Vehicle Tire and Wheel Fitment section to check available specifications by year, make, model, and trim.
Tire Size and Wheel Fitment Work Together
Tire size is only one part of a complete fitment setup. The wheel underneath the tire also has specifications that affect positioning and clearance. Our Tire Size vs Wheel Size guide explains the difference between the two sets of measurements.
Important wheel measurements include wheel width, diameter, offset, backspacing, bolt pattern, and center bore. Visit our Wheel Fitment section to learn how these measurements interact with tire size and vehicle clearance.
If you are moving to a larger wheel while trying to maintain a similar overall tire diameter, read our Plus Sizing Tires & Wheels guide.
Common Reasons Drivers Compare Tire Sizes
Replacing Factory Tires
Confirm the original size and compare available replacement options before purchasing new tires.
Changing Wheel Size
Drivers moving to larger or smaller wheels often compare tire profiles to keep overall diameter reasonably close to the original setup.
Changing Tire Width
A wider or narrower tire can affect clearance and wheel compatibility even when overall tire diameter changes very little.
Browse Tire Sizes
Explore detailed specifications and calculated dimensions for published metric and flotation tire sizes. Each tire-size page provides useful sizing measurements for research and comparison.
Tire Size Guides
How to Read a Tire Size
Understand tire width, aspect ratio, construction, wheel diameter, and common tire markings.
Read Guide →Can You Use a Different Tire Size?
Learn what to check before moving away from the factory tire size.
Read Guide →Tire Size vs Wheel Size
Understand how tire measurements and wheel measurements differ and work together.
Read Guide →Plus Sizing Tires & Wheels
Learn how larger wheels and lower-profile tires can be combined while maintaining a similar overall diameter.
Read Guide →Explore Tire and Wheel Resources
Calculators & Tools
Use tire and wheel calculators to compare dimensions, diameter, speedometer changes, offset, and other fitment measurements.
Vehicle Fitment
Research available tire and wheel specifications by year, make, model, and trim.
Wheel Fitment
Understand wheel diameter, width, offset, backspacing, bolt pattern, center bore, and clearance.
Fitment Guides
Read practical guides about tire sizing, wheels, fitment, measurements, and compatibility.
Verify Tire Size Before Installation
Tire dimensions and calculators are useful research tools, but the final tire choice should match the requirements of your specific vehicle. Confirm tire size, load rating, speed rating, wheel compatibility, inflation requirements, and physical clearance before installation.
For more information about how FitmentCompass researches fitment information, see our Editorial Policy and Research Methodology.