235/45R18 vs 285/40R18
285/40R18 is taller than 235/45R18 by 2.47% — a diameter difference of 0.65″.
What changes between 235/45R18 and 285/40R18
Switching from 235/45R18 to 285/40R18 changes overall diameter by +2.47%, a taller tire by 0.65 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 +1.97″ (235mm → 285mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.33″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -18, 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 | 235/45R18 | 285/40R18 |
|---|---|---|
| Overall diameter | 26.33″ | 26.98″ |
| Section width | 9.25″ (235 mm) | 11.22″ (285 mm) |
| Sidewall height | 4.16″ (45%) | 4.49″ (40%) |
| Rim diameter | 18″ | 18″ |
| Circumference | 82.71″ | 84.75″ |
| Revs per mile | 766 | 748 |
Interactive comparison
275/65R18 or "33" for inch aliases235/45R18
- Diameter
- 26.33″ / 668.7mm
- Section Width
- 9.25″
- Sidewall
- 4.16″ / 105.75mm
- Circumference
- 82.71″
- Revs/Mile
- 766
285/40R18
- Diameter
- 26.98″ / 685.2mm
- Section Width
- 11.22″
- Sidewall
- 4.49″ / 114mm
- Circumference
- 84.75″
- Revs/Mile
- 748
Speedometer & Diameter Impact
Wider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 61.48 mph (98.94 km/h). Your speedometer reads 2.47% low versus actual speed.