265/45R21 vs 315/40R21
315/40R21 is taller than 265/45R21 by 1.75% — a diameter difference of 0.53″.
What changes between 265/45R21 and 315/40R21
Switching from 265/45R21 to 315/40R21 changes overall diameter by +1.75%, a taller tire by 0.53 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 +1.97″ (265mm → 315mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.27″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -12, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.1 mph. Both sizes share the same 21″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 265/45R21 | 315/40R21 |
|---|---|---|
| Overall diameter | 30.39″ | 30.92″ |
| Section width | 10.43″ (265 mm) | 12.4″ (315 mm) |
| Sidewall height | 4.69″ (45%) | 4.96″ (40%) |
| Rim diameter | 21″ | 21″ |
| Circumference | 95.47″ | 97.14″ |
| Revs per mile | 664 | 652 |
Interactive comparison
275/65R18 or "33" for inch aliases265/45R21
- Diameter
- 30.39″ / 771.9mm
- Section Width
- 10.43″
- Sidewall
- 4.69″ / 119.25mm
- Circumference
- 95.47″
- Revs/Mile
- 664
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 61.05 mph (98.25 km/h). Your speedometer reads 1.75% low versus actual speed.