Why the speedo drifts
Your speedometer doesn't measure speed — it counts wheel rotations and multiplies by the rolling circumference it was calibrated for. Change the overall tyre-and-wheel diameter and every rotation covers a different distance, but the speedo keeps using the old number. Taller than stock → speedo under-reads (you're faster than indicated). Shorter → it over-reads.
The simple maths
Overall diameter = (rim size × 25.4) + 2 × (tyre width × aspect ratio). Take a factory 265/60R18: sidewall = 265 × 0.60 = 159 mm, so diameter = 457 + 318 = ≈775 mm.
| Setup | Overall diameter | Variance | Indicated 100 km/h = |
|---|---|---|---|
| 265/60R18 (factory) | ≈775 mm | — | 100 km/h |
| 265/50R20 ✓ done right | ≈773 mm | −0.3% | ≈100 km/h |
| 275/55R20 | ≈811 mm | +4.6% | ≈105 km/h true |
| 285/45R22 | ≈815 mm | +5.2% | ≈105 km/h true |
Same 20" wheel, two very different outcomes — the tyre choice, not the rim, decides whether your speedo stays honest.
The 3% rule
Keep the new overall diameter within ±3% of factory and everything downstream stays happy: speedometer accuracy, gearing feel, and the calibration your ABS and stability systems were tuned around. Beyond it, you're also technically showing incorrect speed — which matters in a country of speed cameras calibrated to true speed while your dash reads low.
Every wheel-and-tyre package we sell is diameter-matched as standard — pair this with our tyre size guide and the numbers take care of themselves.
Quick answers
Do bigger wheels make my speedometer wrong?
Only if the overall rolling diameter changes. A bigger wheel with a proportionally shorter tyre sidewall keeps the same diameter — and the speedo stays accurate. It's the total height that matters, not the rim.
How much variance is acceptable?
The industry rule of thumb is to keep the new rolling diameter within about 3% of factory. Inside that window, speedo error, gearing change and ABS/traction effects are negligible.
My speedo reads 100 — how fast am I actually going with taller tyres?
With tyres 5% taller than stock, an indicated 100 km/h is roughly a true 105 km/h — the speedo under-reads. Shorter tyres do the opposite: it over-reads and you're slower than indicated.
