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Tyre / Wheels Calculator
Switching tyres, wheels — or both? The Tyre tab compares sizes (diameter, speedo error, gearing); the Tyre + Wheels tab adds the wheel specs that decide fitment: J width, offset ET, backspacing, how far it pokes and what happens to inner clearance.
Current tyre
What’s on the car now
New tyre
What you’re considering
Tyre visual — separate & overlaid
Tyre 1 · current
Tyre 2 · new
Overlay
Tyre 2 · new 235/40 R18 · 645 mm Tyre 1 · current 225/45 R17 · 634 mmAxle centre unchanged — the car sits 5.5 mm higher.
+10.9 mm
Diameter change
+1.72%
Overall change
101.72 km/h
Actual at speedo 100
-7.3 mm
Sidewall change
| Spec | Current | New | Difference |
|---|---|---|---|
| Sidewall | 101.3 mm | 94 mm | -7.3 mm |
| Overall diameter | 634.3 mm | 645.2 mm | +10.9 mm |
| Circumference | 1,992.7 mm | 2,027 mm | 34.2 mm |
| Revs per km | 501.8 | 493.4 | -8.5 |
Speedo: you’ll be going 101.72 km/h when the speedo shows 100 — 1.72% faster than indicated.
Gearing: this is like changing your final drive by 1.72% — engine rpm at the same road speed shifts the same amount.
Good to know
- Numbers are theoretical diameters — actual sizes vary slightly between brands and tread depths.
- A rule of thumb: staying within ±3% of the original diameter keeps the speedo, ABS and clearance behaviour predictable.
- Revs per km matter for gearing feel — a shorter tyre spins the engine faster at the same speed.
- Wheel fitment: lower ET and/or wider J pushes the wheel outboard (poke); higher ET pulls it towards the suspension (inner clearance).