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Tire Size Comparator
New wheels, plus-sizing, or a stagger question — compare any two fitments and see diameter change, speedo error and true speed before you order.
Fitment compare · v1.0
±3% rule built in- Current diameter634.3 mm
- New diameter653.2 mm
- Speedo 100 → true103.0 km/h
- VerdictBorderline
How fitment math works
Overall diameter is the number that matters. It's the rim in millimetres plus two sidewalls: rim × 25.4 + 2 × width × aspect ÷ 100. Change the diameter and you change your gearing, your speedometer accuracy, your ground clearance and what your ABS thinks each wheel is doing.
The convention: stay within ±3% of the original diameter. Inside that window, plus-sizing (bigger rim, shorter sidewall) keeps everything honest. Outside it, expect an optimistic speedo with smaller tires, an illegal-in-places pessimistic one with bigger tires, and rubbing risks the calculator can't see — always check arch clearance physically.
Speedo error quick table
| Diameter change | Speedo shows 100 | True speed | Verdict |
|---|---|---|---|
| −3% | 100 | 97.0 | Edge of OK |
| −1% | 100 | 99.0 | Fine |
| 0% | 100 | 100.0 | Perfect |
| +1% | 100 | 101.0 | Fine |
| +3% | 100 | 103.0 | Edge of OK |
FAQ
The common rule is ±3% versus the original fitment. Beyond that, speedometer error grows, gearing changes noticeably, and ABS/stability systems may misread wheel speeds.
The speedometer counts wheel rotations assuming the original circumference. A larger tire covers more ground per rotation, so your true speed is higher than indicated.
In 225/45R17: 225 is the width in millimetres, 45 is the sidewall height as a percentage of width, and 17 is the wheel diameter in inches.