285/30R20 vs 255/35R21
255/35R21 is taller than 285/30R20 by 4.85% — a diameter difference of 1.30″.
What changes between 285/30R20 and 255/35R21
Switching from 285/30R20 to 255/35R21 changes overall diameter by +4.85%, 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 +0.14″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -34, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 68.2 mph. The swap also changes rim diameter from 20″ to 21″, meaning new wheels are required. Wheel cost typically dominates the total upsize budget.
| Spec | 285/30R20 | 255/35R21 |
|---|---|---|
| Overall diameter | 26.73″ | 28.03″ |
| Section width | 11.22″ (285 mm) | 10.04″ (255 mm) |
| Sidewall height | 3.37″ (30%) | 3.51″ (35%) |
| Rim diameter | 20″ | 21″ |
| Circumference | 83.98″ | 88.05″ |
| Revs per mile | 754 | 720 |
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
255/35R21
- Diameter
- 28.03″ / 711.9mm
- Section Width
- 10.04″
- Sidewall
- 3.51″ / 89.25mm
- Circumference
- 88.05″
- Revs/Mile
- 720
Speedometer & Diameter Impact
Different rim — new wheels requiredNarrower tire — verify wheel widthAt an indicated 60 mph, your actual speed is 62.91 mph (101.24 km/h). Your speedometer reads 4.85% low versus actual speed.