285/30R20 vs 255/40R20
255/40R20 is taller than 285/30R20 by 4.86% — a diameter difference of 1.30″.
What changes between 285/30R20 and 255/40R20
Switching from 285/30R20 to 255/40R20 changes overall diameter by +4.86%, a taller tire by 1.30 inches. This sits in the 3–5% caution band, where rubbing at full steering lock or suspension compression becomes possible. Leveling kits or fender-liner trims are often needed on lifted trucks. Section width moves by -1.18″ (285mm → 255mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.65″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -35, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 68.2 mph. Both sizes share the same 20″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 285/30R20 | 255/40R20 |
|---|---|---|
| Overall diameter | 26.73″ | 28.03″ |
| Section width | 11.22″ (285 mm) | 10.04″ (255 mm) |
| Sidewall height | 3.37″ (30%) | 4.02″ (40%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 83.98″ | 88.06″ |
| Revs per mile | 754 | 719 |
Interactive comparison
275/65R18 or "33" for inch aliases285/30R20
- Diameter
- 26.73″ / 679mm
- Section Width
- 11.22″
- Sidewall
- 3.37″ / 85.5mm
- Circumference
- 83.98″
- Revs/Mile
- 754
255/40R20
- Diameter
- 28.03″ / 712mm
- Section Width
- 10.04″
- Sidewall
- 4.02″ / 102mm
- Circumference
- 88.06″
- Revs/Mile
- 719
Speedometer & Diameter Impact
Narrower tire — verify wheel widthAt an indicated 60 mph, your actual speed is 62.92 mph (101.25 km/h). Your speedometer reads 4.86% low versus actual speed.