255/35ZR21 vs 285/35R21
285/35R21 is taller than 255/35ZR21 by 2.95% — a diameter difference of 0.82″.
What changes between 255/35ZR21 and 285/35R21
Switching from 255/35ZR21 to 285/35R21 changes overall diameter by +2.95%, a taller tire by 0.82 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 +1.18″ (255mm → 285mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.42″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -21, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.9 mph. Both sizes share the same 21″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 255/35ZR21 | 285/35R21 |
|---|---|---|
| Overall diameter | 28.03″ | 28.85″ |
| Section width | 10.04″ (255 mm) | 11.22″ (285 mm) |
| Sidewall height | 3.51″ (35%) | 3.93″ (35%) |
| Rim diameter | 21″ | 21″ |
| Circumference | 88.05″ | 90.65″ |
| Revs per mile | 720 | 699 |
Interactive comparison
275/65R18 or "33" for inch aliases255/35ZR21
- Diameter
- 28.03″ / 711.9mm
- Section Width
- 10.04″
- Sidewall
- 3.51″ / 89.25mm
- Circumference
- 88.05″
- Revs/Mile
- 720
285/35R21
- Diameter
- 28.85″ / 732.9mm
- Section Width
- 11.22″
- Sidewall
- 3.93″ / 99.75mm
- Circumference
- 90.65″
- Revs/Mile
- 699
Speedometer & Diameter Impact
Wider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.77 mph (99.41 km/h). Your speedometer reads 2.95% low versus actual speed.