235/40R18 vs 225/45R18
225/45R18 is taller than 235/40R18 by 2.25% — a diameter difference of 0.57″.
What changes between 235/40R18 and 225/45R18
Switching from 235/40R18 to 225/45R18 changes overall diameter by +2.25%, a taller tire by 0.57 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″ (235mm → 225mm), which affects contact patch and fender-well clearance. Sidewall height shifts by +0.29″: a taller sidewall softens ride quality and adds off-road bump protection, with some loss of steering crispness. Revs per mile moves by -17, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.5 mph. Both sizes share the same 18″ rim, so no wheel change is required — this is a pure tire-only swap.
| Spec | 235/40R18 | 225/45R18 |
|---|---|---|
| Overall diameter | 25.4″ | 25.97″ |
| Section width | 9.25″ (235 mm) | 8.86″ (225 mm) |
| Sidewall height | 3.7″ (40%) | 3.99″ (45%) |
| Rim diameter | 18″ | 18″ |
| Circumference | 79.8″ | 81.59″ |
| Revs per mile | 794 | 777 |
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
225/45R18
- Diameter
- 25.97″ / 659.7mm
- Section Width
- 8.86″
- Sidewall
- 3.99″ / 101.25mm
- Circumference
- 81.59″
- Revs/Mile
- 777
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 61.35 mph (98.73 km/h). Your speedometer reads 2.25% low versus actual speed.