245/40R20 vs 255/40R20
255/40R20 is taller than 245/40R20 by 1.14% — a diameter difference of 0.31″.
What changes between 245/40R20 and 255/40R20
Switching from 245/40R20 to 255/40R20 changes overall diameter by +1.14%, a taller tire by 0.31 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″ (245mm → 255mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.16″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -9, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 65.7 mph. Both sizes share the same 20″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 245/40R20 | 255/40R20 |
|---|---|---|
| Overall diameter | 27.72″ | 28.03″ |
| Section width | 9.65″ (245 mm) | 10.04″ (255 mm) |
| Sidewall height | 3.86″ (40%) | 4.02″ (40%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 87.07″ | 88.06″ |
| Revs per mile | 728 | 719 |
Interactive comparison
275/65R18 or "33" for inch aliases245/40R20
- Diameter
- 27.72″ / 704mm
- Section Width
- 9.65″
- Sidewall
- 3.86″ / 98mm
- Circumference
- 87.07″
- Revs/Mile
- 728
255/40R20
- Diameter
- 28.03″ / 712mm
- Section Width
- 10.04″
- Sidewall
- 4.02″ / 102mm
- Circumference
- 88.06″
- Revs/Mile
- 719
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 60.68 mph (97.66 km/h). Your speedometer reads 1.14% low versus actual speed.