205/50R17 vs 235/40R18
235/40R18 is taller than 205/50R17 by 1.32% — a diameter difference of 0.33″.
What changes between 205/50R17 and 235/40R18
Switching from 205/50R17 to 235/40R18 changes overall diameter by +1.32%, a taller tire by 0.33 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 +1.18″ (205mm → 235mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.34″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -10, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 65.9 mph. The swap also changes rim diameter from 17″ to 18″, meaning new wheels are required. Wheel cost typically dominates the total upsize budget.
| Spec | 205/50R17 | 235/40R18 |
|---|---|---|
| Overall diameter | 25.07″ | 25.4″ |
| Section width | 8.07″ (205 mm) | 9.25″ (235 mm) |
| Sidewall height | 4.04″ (50%) | 3.7″ (40%) |
| Rim diameter | 17″ | 18″ |
| Circumference | 78.76″ | 79.8″ |
| Revs per mile | 804 | 794 |
Interactive comparison
275/65R18 or "33" for inch aliases205/50R17
- Diameter
- 25.07″ / 636.8mm
- Section Width
- 8.07″
- Sidewall
- 4.04″ / 102.5mm
- Circumference
- 78.76″
- Revs/Mile
- 804
235/40R18
- Diameter
- 25.4″ / 645.2mm
- Section Width
- 9.25″
- Sidewall
- 3.7″ / 94mm
- Circumference
- 79.8″
- Revs/Mile
- 794
Speedometer & Diameter Impact
Different rim — new wheels requiredWider tire — verify wheel widthAt an indicated 60 mph, your actual speed is 60.79 mph (97.83 km/h). Your speedometer reads 1.32% low versus actual speed.