235/55R20 vs 305/45R20
305/45R20 is taller than 235/55R20 by 2.09% — a diameter difference of 0.63″.
What changes between 235/55R20 and 305/45R20
Switching from 235/55R20 to 305/45R20 changes overall diameter by +2.09%, a taller tire by 0.63 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 +2.76″ (235mm → 305mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.31″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -13, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.4 mph. Both sizes share the same 20″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 235/55R20 | 305/45R20 |
|---|---|---|
| Overall diameter | 30.18″ | 30.81″ |
| Section width | 9.25″ (235 mm) | 12.01″ (305 mm) |
| Sidewall height | 5.09″ (55%) | 5.4″ (45%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 94.8″ | 96.78″ |
| Revs per mile | 668 | 655 |
Interactive comparison
275/65R18 or "33" for inch aliases235/55R20
- Diameter
- 30.18″ / 766.5mm
- Section Width
- 9.25″
- Sidewall
- 5.09″ / 129.25mm
- Circumference
- 94.8″
- Revs/Mile
- 668
305/45R20
- Diameter
- 30.81″ / 782.5mm
- Section Width
- 12.01″
- Sidewall
- 5.4″ / 137.25mm
- Circumference
- 96.78″
- Revs/Mile
- 655
Speedometer & Diameter Impact
Wider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.25 mph (98.58 km/h). Your speedometer reads 2.09% low versus actual speed.