255/35R20 vs 235/40R20
235/40R20 is taller than 255/35R20 by 1.38% — a diameter difference of 0.37″.
What changes between 255/35R20 and 235/40R20
Switching from 255/35R20 to 235/40R20 changes overall diameter by +1.38%, a taller tire by 0.37 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 +0.19″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -10, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 65.9 mph. Both sizes share the same 20″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 255/35R20 | 235/40R20 |
|---|---|---|
| Overall diameter | 27.03″ | 27.4″ |
| Section width | 10.04″ (255 mm) | 9.25″ (235 mm) |
| Sidewall height | 3.51″ (35%) | 3.7″ (40%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 84.91″ | 86.08″ |
| Revs per mile | 746 | 736 |
Interactive comparison
275/65R18 or "33" for inch aliases255/35R20
- Diameter
- 27.03″ / 686.5mm
- Section Width
- 10.04″
- Sidewall
- 3.51″ / 89.25mm
- Circumference
- 84.91″
- Revs/Mile
- 746
235/40R20
- Diameter
- 27.4″ / 696mm
- Section Width
- 9.25″
- Sidewall
- 3.7″ / 94mm
- Circumference
- 86.08″
- Revs/Mile
- 736
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 60.83 mph (97.9 km/h). Your speedometer reads 1.38% low versus actual speed.