255/55R20 vs 305/45R21
305/45R21 is taller than 255/55R20 by 2.46% — a diameter difference of 0.77″.
What changes between 255/55R20 and 305/45R21
Switching from 255/55R20 to 305/45R21 changes overall diameter by +2.46%, a taller tire by 0.77 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 +1.97″ (255mm → 305mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.12″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -16, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.6 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 | 255/55R20 | 305/45R21 |
|---|---|---|
| Overall diameter | 31.04″ | 31.81″ |
| Section width | 10.04″ (255 mm) | 12.01″ (305 mm) |
| Sidewall height | 5.52″ (55%) | 5.4″ (45%) |
| Rim diameter | 20″ | 21″ |
| Circumference | 97.53″ | 99.92″ |
| Revs per mile | 650 | 634 |
Interactive comparison
275/65R18 or "33" for inch aliases255/55R20
- Diameter
- 31.04″ / 788.5mm
- Section Width
- 10.04″
- Sidewall
- 5.52″ / 140.25mm
- Circumference
- 97.53″
- Revs/Mile
- 650
305/45R21
- Diameter
- 31.81″ / 807.9mm
- Section Width
- 12.01″
- Sidewall
- 5.4″ / 137.25mm
- Circumference
- 99.92″
- Revs/Mile
- 634
Speedometer & Diameter Impact
Different rim — new wheels requiredWider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.48 mph (98.94 km/h). Your speedometer reads 2.46% low versus actual speed.