235/50R21 vs 315/40R21
315/40R21 is taller than 235/50R21 by 2.21% — a diameter difference of 0.67″.
What changes between 235/50R21 and 315/40R21
Switching from 235/50R21 to 315/40R21 changes overall diameter by +2.21%, a taller tire by 0.67 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 +0.33″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -15, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.4 mph. Both sizes share the same 21″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 235/50R21 | 315/40R21 |
|---|---|---|
| Overall diameter | 30.25″ | 30.92″ |
| Section width | 9.25″ (235 mm) | 12.4″ (315 mm) |
| Sidewall height | 4.63″ (50%) | 4.96″ (40%) |
| Rim diameter | 21″ | 21″ |
| Circumference | 95.04″ | 97.14″ |
| Revs per mile | 667 | 652 |
Interactive comparison
275/65R18 or "33" for inch aliases235/50R21
- Diameter
- 30.25″ / 768.4mm
- Section Width
- 9.25″
- Sidewall
- 4.63″ / 117.5mm
- Circumference
- 95.04″
- Revs/Mile
- 667
315/40R21
- Diameter
- 30.92″ / 785.4mm
- Section Width
- 12.4″
- Sidewall
- 4.96″ / 126mm
- Circumference
- 97.14″
- Revs/Mile
- 652
Speedometer & Diameter Impact
Wider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.33 mph (98.7 km/h). Your speedometer reads 2.21% low versus actual speed.