245/40R20 vs 275/35R21
275/35R21 is taller than 245/40R20 by 3.11% — a diameter difference of 0.86″.
What changes between 245/40R20 and 275/35R21
Switching from 245/40R20 to 275/35R21 changes overall diameter by +3.11%, a taller tire by 0.86 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″ (245mm → 275mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.07″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -22, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 67.0 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 | 245/40R20 | 275/35R21 |
|---|---|---|
| Overall diameter | 27.72″ | 28.58″ |
| Section width | 9.65″ (245 mm) | 10.83″ (275 mm) |
| Sidewall height | 3.86″ (40%) | 3.79″ (35%) |
| Rim diameter | 20″ | 21″ |
| Circumference | 87.07″ | 89.78″ |
| Revs per mile | 728 | 706 |
Interactive comparison
275/65R18 or "33" for inch aliases245/40R20
- Diameter
- 27.72″ / 704mm
- Section Width
- 9.65″
- Sidewall
- 3.86″ / 98mm
- Circumference
- 87.07″
- Revs/Mile
- 728
275/35R21
- Diameter
- 28.58″ / 725.9mm
- Section Width
- 10.83″
- Sidewall
- 3.79″ / 96.25mm
- Circumference
- 89.78″
- Revs/Mile
- 706
Speedometer & Diameter Impact
Different rim — new wheels requiredWider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.87 mph (99.56 km/h). Your speedometer reads 3.11% low versus actual speed.