235/40R18 vs 255/40R18
255/40R18 is taller than 235/40R18 by 2.48% — a diameter difference of 0.63″.
What changes between 235/40R18 and 255/40R18
Switching from 235/40R18 to 255/40R18 changes overall diameter by +2.48%, 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 +0.79″ (235mm → 255mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.32″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -19, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.6 mph. Both sizes share the same 18″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 235/40R18 | 255/40R18 |
|---|---|---|
| Overall diameter | 25.4″ | 26.03″ |
| Section width | 9.25″ (235 mm) | 10.04″ (255 mm) |
| Sidewall height | 3.7″ (40%) | 4.02″ (40%) |
| Rim diameter | 18″ | 18″ |
| Circumference | 79.8″ | 81.78″ |
| Revs per mile | 794 | 775 |
Interactive comparison
275/65R18 or "33" for inch aliases235/40R18
- Diameter
- 25.4″ / 645.2mm
- Section Width
- 9.25″
- Sidewall
- 3.7″ / 94mm
- Circumference
- 79.8″
- Revs/Mile
- 794
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 61.49 mph (98.96 km/h). Your speedometer reads 2.48% low versus actual speed.