Tire Size Comparison
Enter your current and new tire sizes (width/aspect-rim).
Current tire
New tire
Overall diameter difference
+18.9 mm
+2.98% speedo error
- Current Ø (mm)
- 634
- New Ø (mm)
- 653
How it works
Overall diameter = (rim × 25.4) + 2 × (width × aspect). The % diff is the expected speedometer error.
Formula
Ø = rim(in)×25.4 + 2 × width(mm) × aspect(%)
Worked example
225/45R17 ≈ 634mm vs 245/40R18 ≈ 653mm → +3%.
FAQ
- Acceptable diff?
- ±3% is generally safe and won't confuse the speedometer much.
- ABS impact?
- Yes, if the diff is large — ABS relies on wheel rotation.
- Fuel economy?
- Larger/wider tires slightly increase fuel use.