Tire Size Calculator
Calculate Tire Dimensions & Compare Sizes - Tire Code Decoder & Compatibility Tool
🛞 Pro Tips for Tire Code Reading:
Tire code format: P265/70R17 95H. P = passenger. 265 = width (mm). 70 = aspect ratio (%). R = radial. 17 = wheel diameter (inches). 95 = load index. H = speed rating. Width measured sidewall to sidewall. Diameter calculated: wheel diameter + (2 × section height).
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Tire Reference & Specifications
Load Index Chart:
| Load Index | Weight (lbs) | Weight (kg) | Common Use |
|---|---|---|---|
| 70 | 739 | 335 | Small cars |
| 85 | 1,135 | 515 | Mid-size sedans |
| 95 | 1,520 | 690 | Sedans, SUVs |
| 110 | 2,337 | 1,060 | Light trucks |
| 125 | 3,638 | 1,650 | Heavy trucks |
Speed Rating Chart:
| Rating | Speed (mph) | Speed (km/h) | Use Case |
|---|---|---|---|
| S | 112 | 180 | Temporary/economy |
| T | 118 | 190 | Standard city |
| H | 130 | 210 | Highway standard |
| V | 149 | 240 | Performance |
| W | 168 | 270 | High performance |
| Y | 186 | 300 | Sport/racing |
🛞 Understanding Tire Codes & Specifications
Tire Code Breakdown:
Tire codes contain comprehensive information: vehicle type (P/LT/ST/T), width (millimeters), aspect ratio (%), construction type (R/D/B), wheel diameter (inches), load index, speed rating. Example: P265/70R17 95H. Decoding ensures proper replacement and compatibility. Understanding codes critical for vehicle safety and performance.
Tire Width & Aspect Ratio:- Width: 3-digit number in millimeters. Example: 265 = 265mm width. Wider tires (higher number) better traction, worse fuel economy. Narrower tires (lower number) better efficiency, less traction.
- Aspect Ratio: percentage of width to height. Example: 70 means height is 70% of 265mm. Lower ratios (40-50): shorter sidewalls, sportier feel. Higher ratios (70-80): taller sidewalls, smoother ride.
- Sidewall Height: calculated: width × (aspect ratio ÷ 100). Example: 265 × 0.70 = 185.5mm sidewall height.
- Radial (R): most common modern tires. Cords run across tire perpendicular to direction of travel. Better fuel economy, smoother ride, longer lasting.
- Diagonal (D): older construction. Cords run diagonally. Less common today, generally inferior performance.
- Load Index: 2-3 digit number indicating maximum weight capacity at proper inflation pressure. Example: 95 = 1,520 lbs. Must meet vehicle requirements.
- Wheel Diameter: typically fixed (factory specified). Change limited ±0.5" maximum. Different diameter affects speedometer, odometer, suspension.
- Overall Diameter: calculated from width, ratio, and wheel diameter. Significant differences (>3%) cause issues: speedometer error, suspension problems.
- 3% Rule: recommended maximum difference in overall tire diameter for compatibility and safety. Calculate before installing different size.
- Load Capacity: new tires must equal or exceed original specification. Lower load index = risk of tire failure under vehicle weight.
- Aesthetic Upgrades: larger wheel + lower profile tire maintains similar diameter. Example: 17" with 70% ratio replaced by 19" with 50% ratio.
- Load Changes: load index must increase for towing/heavy use. Larger tires support heavier loads.
- Performance Changes: lower profile = firmer ride, better handling, worse comfort. Higher profile = softer, smoother, less responsive.
- Fuel Economy: larger/wider tires = worse economy. Rolling resistance increases with size and width.
❓ Frequently Asked Questions
How do I know my vehicle's correct tire size?
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Check: vehicle's driver-side door jamb (label sticker), owner's manual, or manufacturer website. Label specifies recommended tire size, load index, and pressure. Factory specification most compatible/safest. If upgrading: consult tire shop. They verify compatibility with suspension, brakes, vehicle weight.
What's a 3% diameter difference rule?
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New tire overall diameter shouldn't differ more than 3% from original. Larger difference: speedometer inaccuracy, odometer error, suspension issues, brake problems. Calculate: (new diameter - old diameter) ÷ old diameter × 100. Example: 31" vs. 30" = 3.3% (slightly over, minimal impact).
Why does tire width affect fuel economy?
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Wider tires: greater rolling resistance (more friction with road). More energy needed to move vehicle forward. Result: worse fuel economy (typically 3-5% per 10mm width increase). Narrower tires: less resistance, better economy. Trade-off: narrower = less traction, especially cornering/wet conditions.
What's the difference between tire and wheel size?
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Wheel: metal rim (16", 17", 18" etc.). Tire: rubber outer covering on wheel. Tire size includes wheel diameter as final number. Example: 265/70R17 = 17" wheel. Upgrading often means larger wheel + lower profile tire (maintain similar overall diameter).
Can I use all-season instead of winter tires?
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Not recommended in snow/ice. Winter tires: special rubber compound stays flexible in cold (better traction). All-season: hardens in cold, poor grip. Different handling characteristics. Safety issue: winter tires required in snow-heavy areas legally. All-season acceptable spring-fall in mild climates.
How old should tires be before replacement?
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Age + tread wear matters. Tread depth: minimum 2/32" legally (4/32" safer). Age: 10 years maximum (rubber deteriorates). Check DOT code for production date. Older tires (6-10 years): check carefully for dry rot. Storage affects aging. Driving in extreme heat/UV accelerates deterioration.
Why is tire pressure important for load index?
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Load index rating assumes correct tire pressure. Underinflated: less load capacity, excessive heat generation, tire failure risk. Overinflated: also reduces capacity, affects handling. Proper pressure critical. Check vehicle door label, not tire sidewall. Pressure changes with temperature (±5 PSI per 50°F change).