275/30ZR20 vs 245/40R20
245/40R20 is taller than 275/30ZR20 by 4.61% — a diameter difference of 1.22″.
What changes between 275/30ZR20 and 245/40R20
Switching from 275/30ZR20 to 245/40R20 changes overall diameter by +4.61%, a taller tire by 1.22 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″ (275mm → 245mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.61″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -33, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 68.0 mph. Both sizes share the same 20″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 275/30ZR20 | 245/40R20 |
|---|---|---|
| Overall diameter | 26.5″ | 27.72″ |
| Section width | 10.83″ (275 mm) | 9.65″ (245 mm) |
| Sidewall height | 3.25″ (30%) | 3.86″ (40%) |
| Rim diameter | 20″ | 20″ |
| Circumference | 83.24″ | 87.07″ |
| Revs per mile | 761 | 728 |
Interactive comparison
275/65R18 or "33" for inch aliases275/30ZR20
- Diameter
- 26.5″ / 673mm
- Section Width
- 10.83″
- Sidewall
- 3.25″ / 82.5mm
- Circumference
- 83.24″
- Revs/Mile
- 761
245/40R20
- Diameter
- 27.72″ / 704mm
- Section Width
- 9.65″
- Sidewall
- 3.86″ / 98mm
- Circumference
- 87.07″
- Revs/Mile
- 728
Speedometer & Diameter Impact
Narrower tire — verify wheel widthAt an indicated 60 mph, your actual speed is 62.76 mph (101.01 km/h). Your speedometer reads 4.61% low versus actual speed.