Compare tire sizes, calculate diameter, width, and speedometer error when changing tire dimensions.
Reading Tire Size
A tire marked 225/45R17 means: 225mm width, 45% aspect ratio (sidewall height = 45% of width), R = Radial, 17" rim diameter.
Safe Tire Size Change Guidelines
- Stay within Β±3% overall diameter to avoid speedometer errors and clearance issues
- Wider tires = better grip but higher rolling resistance and fuel consumption
- Lower profile = stiffer ride, better handling, but more susceptible to pothole damage
How to Use This Tire Size Calculator
Enter your current and proposed tire sizes (width/aspect ratio/rim diameter format). The calculator compares diameters, circumferences, speedometer error, and ground clearance.
Formula & How It Works
Tire Diameter = Rim + 2 Γ Sidewall. Sidewall = Width (mm) Γ Aspect Ratio / 25.4. Speedometer error = (New Diameter / Old Diameter β 1) Γ 100%.
Calculation Example
Upgrading from 225/65R17 to 245/65R17: Diameter increases 0.55" (+1.9%). Speedometer at 60 mph reads 58.9 mph actual. Ground clearance increases 0.28".
Expert Tips
Keep speedometer error under 3% to avoid legal issues and maintain accurate speed readings. Larger tires reduce acceleration and fuel economy but improve ground clearance and ride comfort. Check for clearance issues before upsizing.