Understanding Tire Sizes
How to Read Tire Sizes
Tire sizes are typically written in the format: 205/55R16
- 205 - Section width in millimeters (tire width from sidewall to sidewall)
- 55 - Aspect ratio (sidewall height as percentage of width)
- R - Radial construction (nearly all modern tires)
- 16 - Wheel diameter in inches (rim size)
Tire Dimensions Explained
Sidewall Height: Calculated as (Width ร Aspect Ratio) รท 100. For 205/55R16: (205 ร 55) รท 100 = 112.75mm
Overall Diameter: (Sidewall Height ร 2) + (Rim Diameter ร 25.4mm). This determines speedometer accuracy and ground clearance.
Circumference: Diameter ร ฯ (3.14159). Determines how far the tire travels in one revolution.
Why Tire Size Matters
- Speedometer Accuracy: Different diameter affects speed readings. Most manufacturers allow ยฑ3% difference.
- Vehicle Clearance: Larger tires may rub on fenders or suspension components
- Fuel Economy: Larger/wider tires increase rolling resistance
- Handling: Width and sidewall height affect grip and steering response
- Ride Comfort: Taller sidewalls provide more cushioning
Plus Sizing (Upgrading Wheel Size)
Plus sizing means fitting larger wheels while maintaining similar overall tire diameter. For example, changing from 205/55R16 to 225/45R17 keeps diameter similar while adding 1 inch to wheel size.
Benefits: Improved handling, better looks, enhanced braking
Drawbacks: Harsher ride, higher cost, potential speedometer error
Acceptable Tire Size Changes
- Diameter difference: Keep within 3% of original for accurate speedometer
- Width increase: Usually 10-20mm wider is safe, but check clearance
- Aspect ratio: Lower ratio = sportier handling but harsher ride
- Load index: Never go lower than original equipment specifications
Common Tire Size Upgrades
From 205/55R16: Common upgrades include 215/50R17, 225/45R17, or 205/50R17
From 225/45R17: Can upgrade to 235/40R18, 245/35R19, or stick with 225/40R18
From 195/65R15: Popular changes are 205/60R16 or 205/55R16
Speedometer Error Impact
If your new tire has a 5% larger diameter, your speedometer will read lower than actual speed. At an indicated 60 mph, you'd actually be traveling 63 mph. Most jurisdictions require speedometers to be accurate within 10%, but it's best to stay within 3%.
When to Replace Tires
- Tread depth below 2mm (legal limit in UK is 1.6mm, but replace before this)
- Visible cracks, bulges, or damage to sidewalls
- Uneven wear patterns indicating alignment or suspension issues
- Age over 6 years regardless of tread depth
- Vibration or noise that wasn't present before
Tire Maintenance Tips
- Check tire pressure monthly (including spare)
- Rotate tires every 5,000-8,000 miles for even wear
- Have wheel alignment checked if vehicle pulls to one side
- Inspect for damage, nails, or foreign objects regularly
- Replace all four tires at once for best performance (or at least in pairs)