Tire Size Calculator
Calculate tire dimensions and speedometer error when changing wheel sizes
New Tire Size
Format: Width/AspectRatio — e.g. 205/55R16
Width (mm)
Aspect %
Rim (in)
Stock/OEM Tire (for comparison)
Leave blank if not comparing
Width (mm)
Aspect %
Rim (in)
Tire Dimensions
Overall Diameter
—
Sidewall Height
—
Circumference
—
Revolutions/km
—
Reading Tire Size Numbers
A tire marked 205/55R16 means: 205 mm section width, 55% aspect ratio (sidewall height = 55% of 205 = 112.75 mm), R = radial, 16 inch rim diameter. Overall diameter = rim (in mm) + 2 × sidewall = 406 + 225.5 = 631.5 mm.
Fitting larger tires without recalibration causes speedometer under-reading (actual speed > indicated). Larger diameter tires also affect odometer, fuel trim, and ABS calibration. The general rule: stay within ±3% overall diameter of OEM spec to avoid issues with modern ECU systems.
lightbulb Example
205/55R16 vs 215/55R17:
1Stock dia: 631 mm → New: 672 mm
2Speed error: +6.5% (actual > indicated)
✓ Too large — stay within ±3%