225/50R18 vs 235/40R20
235/40R20 is taller than 225/50R18 by 2.02% — a diameter difference of 0.54″.
What changes between 225/50R18 and 235/40R20
Switching from 225/50R18 to 235/40R20 changes overall diameter by +2.02%, a taller tire by 0.54 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″ (225mm → 235mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.73″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -15, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 66.3 mph. The swap also changes rim diameter from 18″ to 20″, meaning new wheels are required. Wheel cost typically dominates the total upsize budget.
| Spec | 225/50R18 | 235/40R20 |
|---|---|---|
| Overall diameter | 26.86″ | 27.4″ |
| Section width | 8.86″ (225 mm) | 9.25″ (235 mm) |
| Sidewall height | 4.43″ (50%) | 3.7″ (40%) |
| Rim diameter | 18″ | 20″ |
| Circumference | 84.38″ | 86.08″ |
| Revs per mile | 751 | 736 |
Interactive comparison
275/65R18 or "33" for inch aliases225/50R18
- Diameter
- 26.86″ / 682.2mm
- Section Width
- 8.86″
- Sidewall
- 4.43″ / 112.5mm
- Circumference
- 84.38″
- Revs/Mile
- 751
235/40R20
- Diameter
- 27.4″ / 696mm
- Section Width
- 9.25″
- Sidewall
- 3.7″ / 94mm
- Circumference
- 86.08″
- Revs/Mile
- 736
Speedometer & Diameter Impact
Different rim — new wheels requiredAt an indicated 60 mph, your actual speed is 61.21 mph (98.51 km/h). Your speedometer reads 2.02% low versus actual speed.