Tire Size Calculator Online Free Tool
Tire Size Calculator
Tire Code
Width / Aspect Ratio Construction Wheel Diameter
Tire Code Reference
Tire Dimensions
Calculation Formula:
Understanding Tire Code: Complete Guide to Tire Size
Learning how to read tire size is essential for any vehicle owner. The tire code is an alphanumeric designation printed on the sidewall of every tire that contains crucial information about the tire's dimensions, construction, and performance capabilities. Our tire size calculator helps you decode this information and understand what each number and letter means for your vehicle's performance, safety, and compatibility.
Breaking Down Each Component of the Tire Code
1. Intended Use (Vehicle Class)
The first letter(s) indicate the tire type and vehicle class. P stands for passenger car tires designed for cars, SUVs, crossovers, minivans, and small pickup trucks. LT designates light truck tires built for heavy loads and trailer towing. ST indicates special trailer tires for boat, utility, and travel trailers. T marks temporary spare tires not intended for regular use. Understanding your vehicle tire requirements ensures you select appropriate tires.
2. Nominal Section Width (Tire Width)
The three-digit number following the vehicle class represents the tire width in millimeters, measured from sidewall to sidewall at the widest point. For example, in tire size 215/65R16, the "215" means the nominal section width is 215mm (approximately 8.5 inches). Wider tires generally provide better traction and handling but may reduce fuel efficiency. Our tire width calculator helps you understand how different widths affect performance and whether they're compatible with your vehicle.
3. Aspect Ratio (Sidewall Height Percentage)
The two-digit number after the forward slash is the tire aspect ratio, expressing sidewall height as a percentage of tire width. In 215/65R16, the "65" means the sidewall height is 65% of 215mm = 139.75mm. Lower aspect ratios (like 40 or 45) create shorter sidewalls for sportier handling and appearance, while higher aspect ratios (65-75) provide more cushioning and comfortable ride quality. Understanding how to calculate tire height is crucial when considering different sizes.
4. Internal Construction Type
The single letter indicates tire construction method. R (Radial) is standard for virtually all modern passenger vehicles, with cords running perpendicular to the direction of travel, providing better fuel efficiency, handling, and heat dissipation. D (Diagonal/Bias) features cords at 30-40° angles, now rare except in some specialty applications. B (Bias-Belted) adds stabilizer belts above diagonal plies. Radial tire advantages include longer tread life and improved performance compared to older bias-ply designs.
5. Wheel Diameter (Rim Size)
The one or two-digit number after construction type specifies wheel diameter in inches. In 215/65R16, "16" means the tire fits a 16-inch diameter wheel. Common wheel sizes range from 14 to 22 inches for passenger vehicles, with larger sizes (18"+) increasingly popular for aesthetic and performance reasons. When using our tire size comparison calculator, ensure the tire's internal diameter matches your wheel size exactly—this dimension cannot vary.
6. Load Index (Weight Capacity)
The two or three-digit tire load index indicates maximum weight each tire can safely support at proper inflation. For example, load index 95 means 1,520 lbs (690 kg) capacity per tire at recommended PSI. Always match or exceed your vehicle's original tire load rating. Load index chart values range from 70 (739 lbs) to 130 (4,189 lbs) for passenger vehicles. Insufficient load capacity risks tire failure, especially when carrying heavy loads or towing.
7. Speed Rating (Maximum Speed)
The final letter is the tire speed rating, indicating maximum sustained speed capacity. Common ratings include: S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (168 mph), Y (186 mph), and Z (149+ mph). The speed rating applies when the tire carries its maximum load at proper inflation. Always match or exceed your vehicle's OEM tire speed rating. Higher-rated tires typically offer better handling and heat resistance but may sacrifice ride comfort and tread life.
How to Calculate Tire Size: Step-by-Step Formula
Understanding tire size calculation helps you determine actual dimensions from the tire code and assess compatibility when considering alternatives. Our tire dimension calculator automates these calculations, but knowing the formulas helps you understand why certain sizes work together and others don't.
Essential Tire Size Formulas:
Practical Example: Calculating 265/70R17
These calculations are essential for tire fitment, understanding speedometer accuracy, and determining compatible tire sizes. Use our tire size converter to instantly calculate dimensions and compare alternatives.
Changing Tire and Wheel Size: What You Need to Know
Many drivers consider changing tire size or upgrading to larger rims for aesthetic appeal, improved handling, or specific performance characteristics. While it's possible to safely use different tire sizes, there are critical factors to consider to avoid negative impacts on vehicle safety, performance, and mechanical systems. Our tire fitment calculator helps ensure compatibility.
The 3% Rule: Golden Standard for Tire Replacement
Industry Standard: When selecting alternative tire sizes, the overall diameter should remain within ±3% of your original tire diameter. This tolerance minimizes impacts on:
- Speedometer and odometer accuracy
- Anti-lock brake system (ABS) calibration
- Traction control and stability control systems
- All-wheel drive (AWD) torque distribution
- Transmission shift points and gear ratios
- Fuel efficiency calculations
Plus-Sizing: Larger Wheels, Lower Profile Tires
Plus-sizing (also called inch-up or plus-one/plus-two fitment) involves installing larger diameter wheels with lower aspect ratio tires to maintain similar overall diameter. For example, going from 16-inch to 17-inch wheels (plus-one) or 18-inch wheels (plus-two).
Benefits of Plus-Sizing
- Improved handling and cornering response
- Enhanced brake clearance for larger rotors
- Better aesthetic appearance
- Reduced sidewall flex for sportier feel
- Increased contact patch potential
- Lower risk of hydroplaning
Drawbacks to Consider
- Harsher ride quality from less cushioning
- Higher tire and wheel costs
- Increased road noise
- Greater susceptibility to wheel/tire damage
- Potential rubbing on wheel wells
- Reduced fuel economy from added weight
Speedometer and Odometer Impact
Tire diameter affects speedometer accuracy because speedometers calculate speed based on wheel rotation frequency. Larger diameter tires cover more distance per revolution, causing speedometer to read lower than actual speed. Conversely, smaller tires cause speedometer to read higher than actual.
Real-World Example:
Critical Considerations When Changing Tire Size
1. Wheel Well Clearance
Ensure adequate clearance between tires and fender liners, suspension components, and frame rails. Test at full steering lock and with suspension compressed. Rubbing causes tire damage and can affect steering.
2. Load Capacity Requirements
New tires must equal or exceed original load index rating. Insufficient capacity risks catastrophic failure, especially when towing or carrying heavy loads. Check your vehicle's door jamb sticker for required ratings.
3. Matching Tires on Same Axle
Always use matching tire sizes on the same axle. Mismatched sizes cause handling imbalances, premature wear, and stress on differentials. For AWD vehicles, all four tires should be within 2/32" tread depth to prevent drivetrain damage.
4. TPMS (Tire Pressure Monitoring System)
New wheels require compatible TPMS sensors. Sensors must be transferred or new ones installed and programmed to your vehicle. Improper TPMS setup triggers warning lights and prevents pressure monitoring.
5. Speedometer Recalibration
Some vehicles allow speedometer recalibration through dealer programming or aftermarket modules. This restores accuracy after significant tire size changes and may be required to pass inspection in some jurisdictions.
Tire Replacement Best Practices and Safety Tips
Whether replacing tires due to wear and tear, tire blowout, or seasonal changes, following best practices ensures optimal safety, performance, and tire longevity. Understanding when to replace tires and how to choose replacement tires protects your investment and keeps you safe on the road.
When to Replace Tires
Tread Depth Indicators
- 2/32" minimum legal tread depth (replace immediately)
- 4/32" for wet conditions (recommended minimum)
- 6/32" for winter/snow tires in snowy climates
- Use penny test: Insert penny with Lincoln's head down—if you see his entire head, replace tires
- Check tread wear indicators (raised bars in grooves)
Age and Condition Signs
- 6 years old: Have professionally inspected annually
- 10 years old: Replace regardless of appearance
- Sidewall cracks or bulges (replace immediately)
- Uneven wear patterns (alignment/suspension issues)
- Vibration or noise changes while driving
- Frequent air loss or pressure problems
Replacement Strategies
Essential Tire Maintenance Tips
Proper Inflation
Check tire pressure monthly and before long trips. Use vehicle's recommended PSI (door jamb sticker), not tire sidewall max. Underinflation causes excess heat, wear, and reduced fuel economy. Overinflation reduces traction and causes harsh ride.
Regular Rotation
Rotate tires every 5,000-7,000 miles to promote even wear. Follow vehicle-specific rotation pattern (usually front-to-back, cross pattern, or rearward cross). Regular rotation extends tire life by 20-30% and maintains balanced handling.
Alignment Checks
Get wheel alignment checked annually or after hitting potholes/curbs. Misalignment causes rapid uneven wear, pulling to one side, and reduced fuel efficiency. Signs include off-center steering wheel or vehicle drift.
Key Takeaways: Master Tire Sizes
Decode Tire Code Accurately
Understanding tire size numbers empowers informed decisions. Each component—width, aspect ratio, construction, diameter, load index, speed rating—affects performance, safety, and compatibility. Use our calculator to decode any tire size instantly.
Calculate Before You Buy
Always calculate tire dimensions before purchasing alternatives. Section height, overall diameter, and circumference determine fitment and speedometer accuracy. Our tire comparison tool shows exact differences between sizes.
Follow the 3% Rule
Keep replacement tire diameter within ±3% of original to maintain speedometer accuracy, preserve ABS and traction control calibration, and avoid mechanical issues. This industry standard protects vehicle systems and safety.
Consider All Impacts
Changing tire size affects ride quality, handling, fuel economy, speedometer readings, and appearance. Larger wheels need lower profile tires to maintain diameter. Balance aesthetic goals with practical performance needs.
Replace Strategically
Replace all four tires when possible, especially for AWD vehicles. If replacing pairs, put new tires on rear axle for stability. Match size, type, and brand for optimal performance and safety. Check tread depth and age regularly.
Maintain Properly
Check tire pressure monthly, rotate every 5,000-7,000 miles, inspect for damage regularly, and check alignment annually. Proper maintenance extends tire life 20-30% and ensures safe operation. Replace tires at 2/32" tread or 10 years age.