245/40R18 vs 255/40R18
255/40R18 is taller than 245/40R18 by 1.22% — a diameter difference of 0.31″.
What changes between 245/40R18 and 255/40R18
Switching from 245/40R18 to 255/40R18 changes overall diameter by +1.22%, a taller tire by 0.31 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 +0.39″ (245mm → 255mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.16″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -9, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 65.8 mph. Both sizes share the same 18″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 245/40R18 | 255/40R18 |
|---|---|---|
| Overall diameter | 25.72″ | 26.03″ |
| Section width | 9.65″ (245 mm) | 10.04″ (255 mm) |
| Sidewall height | 3.86″ (40%) | 4.02″ (40%) |
| Rim diameter | 18″ | 18″ |
| Circumference | 80.79″ | 81.78″ |
| Revs per mile | 784 | 775 |
Interactive comparison
275/65R18 or "33" for inch aliases245/40R18
- Diameter
- 25.72″ / 653.2mm
- Section Width
- 9.65″
- Sidewall
- 3.86″ / 98mm
- Circumference
- 80.79″
- Revs/Mile
- 784
255/40R18
- Diameter
- 26.03″ / 661.2mm
- Section Width
- 10.04″
- Sidewall
- 4.02″ / 102mm
- Circumference
- 81.78″
- Revs/Mile
- 775
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 60.73 mph (97.74 km/h). Your speedometer reads 1.22% low versus actual speed.