265/40R21 vs 255/45R21
255/45R21 is taller than 265/40R21 by 2.35% — a diameter difference of 0.69″.
What changes between 265/40R21 and 255/45R21
Switching from 265/40R21 to 255/45R21 changes overall diameter by +2.35%, a taller tire by 0.69 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 -0.39″ (265mm → 255mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.35″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -16, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.5 mph. Both sizes share the same 21″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 265/40R21 | 255/45R21 |
|---|---|---|
| Overall diameter | 29.35″ | 30.04″ |
| Section width | 10.43″ (265 mm) | 10.04″ (255 mm) |
| Sidewall height | 4.17″ (40%) | 4.52″ (45%) |
| Rim diameter | 21″ | 21″ |
| Circumference | 92.19″ | 94.36″ |
| Revs per mile | 687 | 671 |
Interactive comparison
275/65R18 or "33" for inch aliases265/40R21
- Diameter
- 29.35″ / 745.4mm
- Section Width
- 10.43″
- Sidewall
- 4.17″ / 106mm
- Circumference
- 92.19″
- Revs/Mile
- 687
255/45R21
- Diameter
- 30.04″ / 762.9mm
- Section Width
- 10.04″
- Sidewall
- 4.52″ / 114.75mm
- Circumference
- 94.36″
- Revs/Mile
- 671
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 61.41 mph (98.83 km/h). Your speedometer reads 2.35% low versus actual speed.