The 235/40R18 is a common tire size for performance and sports cars such as BMW 3 Series, Volkswagen GTI, and Audi A4.
With a 94.0 mm sidewall (40% aspect ratio), this low-profile tire delivers sharp steering response and precise cornering grip, though the ride feels firmer over rough surfaces.
Compared to the baseline 235/45R18, this tire's overall diameter differs by -3.5% — a speedometer calibrated for 235/45R18 would read 103.6 km/h at an actual 100 km/h.
When replacing, verify that your wheel well can accommodate the 645.2 mm overall diameter. Keep speedometer deviation within ±3% for safety and legal compliance.
| Tire Code | 235/40R18 |
| Section Width | 235 mm |
| Aspect Ratio | 40% |
| Rim Diameter | 18" |
| Sidewall Height | 94.0 mm |
| Overall Diameter | 645.2 mm / 25.40" |
| Circumference | 2027.0 mm / 79.80" |
| Revolutions per km | 493.4 |
| Revolutions per mile | 794.0 |
| Tire Code | OD (mm) | Δ mm | Δ % | Speedo at 100 km/h |
|---|---|---|---|---|
| 225/40R18 | 637.2 | -8.0 | -1.2% | 101.3 |
| 235/40R18 (current) | 645.2 | — | — | 100.0 |
| 245/40R18 | 653.2 | +8.0 | +1.2% | 98.8 |
| 235/45R18 | 668.7 | +23.5 | +3.6% | 96.5 |
| 235/40R19 | 670.6 | +25.4 | +3.9% | 96.2 |
| 235/55R18 | 715.7 | +70.5 | +10.9% | 90.1 |