255/40R20 vs 235/45R20
235/45R20 is taller than 255/40R20 by 1.05% — a diameter difference of 0.30″.
What changes between 255/40R20 and 235/45R20
Switching from 255/40R20 to 235/45R20 changes overall diameter by +1.05%, a taller tire by 0.30 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 -0.79″ (255mm → 235mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.14″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -7, 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 | 255/40R20 | 235/45R20 |
|---|---|---|
| Overall diameter | 28.03″ | 28.33″ |
| Section width | 10.04″ (255 mm) | 9.25″ (235 mm) |
| Sidewall height | 4.02″ (40%) | 4.16″ (45%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 88.06″ | 88.99″ |
| Revs per mile | 719 | 712 |
Interactive comparison
275/65R18 or "33" for inch aliases255/40R20
- Diameter
- 28.03″ / 712mm
- Section Width
- 10.04″
- Sidewall
- 4.02″ / 102mm
- Circumference
- 88.06″
- Revs/Mile
- 719
235/45R20
- Diameter
- 28.33″ / 719.5mm
- Section Width
- 9.25″
- Sidewall
- 4.16″ / 105.75mm
- Circumference
- 88.99″
- Revs/Mile
- 712
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 60.63 mph (97.58 km/h). Your speedometer reads 1.05% low versus actual speed.