235/55R20 vs 315/40R21
315/40R21 is taller than 235/55R20 by 2.47% — a diameter difference of 0.74″.
What changes between 235/55R20 and 315/40R21
Switching from 235/55R20 to 315/40R21 changes overall diameter by +2.47%, a taller tire by 0.74 inches. This lands inside the ±3% fitment safe zone, so most vehicles will accept the swap without rubbing, speedometer recalibration, or ABS/TPMS complications. Section width moves by +3.15″ (235mm → 315mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.13″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -16, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.6 mph. The swap also changes rim diameter from 20″ to 21″, meaning new wheels are required. Wheel cost typically dominates the total upsize budget.
| Spec | 235/55R20 | 315/40R21 |
|---|---|---|
| Overall diameter | 30.18″ | 30.92″ |
| Section width | 9.25″ (235 mm) | 12.4″ (315 mm) |
| Sidewall height | 5.09″ (55%) | 4.96″ (40%) |
| Rim diameter | 20″ | 21″ |
| Circumference | 94.8″ | 97.14″ |
| Revs per mile | 668 | 652 |
Interactive comparison
275/65R18 or "33" for inch aliases235/55R20
- Diameter
- 30.18″ / 766.5mm
- Section Width
- 9.25″
- Sidewall
- 5.09″ / 129.25mm
- Circumference
- 94.8″
- Revs/Mile
- 668
315/40R21
- Diameter
- 30.92″ / 785.4mm
- Section Width
- 12.4″
- Sidewall
- 4.96″ / 126mm
- Circumference
- 97.14″
- Revs/Mile
- 652
Speedometer & Diameter Impact
Different rim — new wheels requiredWider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.48 mph (98.94 km/h). Your speedometer reads 2.47% low versus actual speed.