Tire Size Calculator
Calculate Tire Diameter
Enter tire size in standard format (e.g., 235/65R17)
Compare Two Tire Sizes
Compare original vs new tire specifications
Calculate Sidewall Height
Determine tire sidewall height and overall dimensions
Tire RPM Calculator
Calculate tire RPM at given speed
Tire Specifications
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Tire size format: Width/AspectRatio R RimDiameter (e.g., 235/65R17). Width is in millimeters, aspect ratio is sidewall height as percentage of width, R means radial, and rim diameter is in inches. Accurate tire sizing is important for safety, speedometer accuracy, and fuel economy.
Complete Tire Size Guide
Understanding tire sizes is essential for vehicle maintenance, safety, and performance. This guide explains tire specifications, how to read tire codes, and how to select the right tires for your vehicle.
Understanding Tire Size Format
A tire size like 235/65R17 108H breaks down as follows:
- 235: Tire width in millimeters
- 65: Aspect ratio (sidewall height as % of width). 65% means sidewall is 235mm × 0.65 = 153mm
- R: Radial construction (standard for all modern tires)
- 17: Rim diameter in inches
- 108: Load index (maximum weight capacity)
- H: Speed rating (H = 210 km/h maximum)
Common Tire Sizes & Vehicles
| Tire Size | Diameter (inches) | Diameter (mm) | Common Vehicle |
|---|---|---|---|
| 175/70R13 | 22.4" | 569 mm | Hatchback, compact car |
| 185/65R15 | 24.0" | 610 mm | Sedan, compact car |
| 205/55R16 | 24.9" | 632 mm | Mid-size sedan |
| 225/60R16 | 25.9" | 658 mm | SUV, crossover |
| 235/65R17 | 27.1" | 688 mm | SUV, truck |
| 245/75R16 | 28.6" | 727 mm | Full-size truck |
Speed Ratings Explained
| Rating | Speed (km/h) | Speed (mph) | Best For |
|---|---|---|---|
| Q | 160 | 100 | Temporary/winter tires |
| R | 170 | 106 | Light trucks, trailers |
| S | 180 | 112 | Older sedans, family cars |
| T | 190 | 118 | Most family sedans |
| H | 210 | 130 | Modern sedans, crossovers |
| V | 240 | 149 | Performance vehicles |
| W | 270 | 168 | High-performance cars |
| Y | 300 | 186 | Exotic/sports cars |
Tire Diameter Calculation Formula
For tire 235/65R17:
Sidewall Height = 235mm × 0.65 = 152.75mm = 6.01 inches
Total Diameter = 17 + (2 × 6.01) = 17 + 12.02 = 29.02 inches
Or in metric:
Rim Diameter = 17 × 25.4mm = 431.8mm
Total = 431.8 + (2 × 152.75) = 737.3mm
Load Index Guide
| Load Index | Weight (lbs) | Weight (kg) |
|---|---|---|
| 75 | 853 | 387 |
| 82 | 1047 | 475 |
| 96 | 1565 | 710 |
| 108 | 2337 | 1060 |
| 120 | 3527 | 1600 |
Important Tire Care Tips
- Check Pressure Monthly: Incorrect tire pressure affects fuel economy and tire life. Check when tires are cold (3+ hours after driving)
- Rotate Every 8,000 km: Helps tires wear evenly. Most vehicles can go 40,000-80,000 km per tire set
- Wheel Alignment: Misalignment causes uneven wear and poor handling. Check annually
- Balance Tires: Unbalanced tires cause vibration and uneven wear. Balance when installing new tires
- Replace at 2/32 Tread: Tread depth affects wet traction. Legal minimum 2/32", but safety experts recommend 4/32"
- Avoid Overloading: Exceeding load rating damages tires and reduces safety. Check vehicle door jamb for proper tire size
Tire Size Change Guidelines
- Rim Size Change: Can change to different rim size if new tire's overall diameter ±3% matches original
- Width Change: Width can vary ±25mm without major impact on speedometer accuracy
- Aspect Ratio Change: Lowering aspect ratio (e.g., 65 to 55) requires larger rim diameter to maintain overall diameter
- Speedometer Impact: Larger tires result in speedometer reading lower than actual speed. 3% change = 3 mph error at 100 mph
- Performance Impact: Wider, shorter sidewall tires improve handling but reduce comfort. Narrower tires better for snow/ice
When to Replace Tires
- Tread Depth 2/32": Legal minimum, but not safe. Replace at 4/32" or when tread bars visible
- Age: Tires degrade with age. Replace every 5-10 years regardless of tread depth
- Damage: Bulges, cracks, cuts larger than 6mm, or punctures not repairable
- Uneven Wear: One side worn more than other indicates alignment or pressure problem
- Vibration or Noise: May indicate tire damage, imbalance, or misalignment
- Sidewall Separation: Bulge in sidewall means structural failure. Safety hazard—replace immediately
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Frequently Asked Questions
1. What does each number in a tire size mean?
235/65R17: 235 is width (mm), 65 is aspect ratio (sidewall height %), R is radial, 17 is rim size (inches). Additional codes: 108 is load index, H is speed rating (210 km/h max).
2. Can I use a different tire size than recommended?
Yes, but it must have same overall diameter ±3%. Check vehicle owner's manual or door jamb for approved sizes. Changing tire size affects speedometer accuracy, fuel economy, and handling.
3. How is tire diameter calculated?
Diameter = Rim Diameter + (2 × Sidewall Height). For 235/65R17: Sidewall = 235 × 0.65 = 153mm = 6.01". Total = 17 + 12.02 = 29.02 inches.
4. What does the load index mean?
Load index is maximum weight per tire. 108 = 1060kg (2337 lbs). Never use tires below recommended load index. Overloading damages tires and reduces safety.
5. What's the difference between all-season and winter tires?
All-season: Good for most conditions, moderate performance. Winter: Better grip in snow/ice with softer rubber, deeper tread, thinner sipes. Winter tires significantly improve snow performance.
6. How often should I rotate tires?
Every 8,000-10,000 km or every oil change. Rotation helps tires wear evenly, extending life 10,000-15,000 km. Essential for even tire wear.
7. When should I replace tires?
At 2/32" tread depth (legal minimum), but 4/32" recommended for safety. Also replace if older than 10 years, have visible damage, bulges, or uneven wear despite alignment.
8. Does tire size affect speedometer?
Yes. Larger tires make speedometer read slower than actual speed. A 3% diameter increase = speedometer reads 3 km/h low at 100 km/h. Smaller tires cause opposite effect.
9. What does speed rating mean?
Speed rating (H, V, W, Y) indicates maximum safe speed. H = 210 km/h, V = 240 km/h, W = 270 km/h, Y = 300 km/h. Use tires matching vehicle capability.
10. How do I check tire tread depth?
Penny test: Insert penny head-down into tread. If you see top of head, tread is too shallow (2/32" or less). Digital gauges also available. Check multiple locations.
11. What's the correct tire pressure?
Check vehicle door jamb or owner's manual for proper PSI (usually 30-35 PSI). Check when tires are cold (3+ hours after driving or before driving). Pressure increases 1 PSI per 6°C temperature rise.
12. Can wider tires improve fuel economy?
No, wider tires typically reduce fuel economy due to increased rolling resistance. Narrower tires generally improve fuel economy 1-3%. Keep to manufacturer's recommended size.