215/45R17 vs 235/40R18
235/40R18 is taller than 215/45R17 by 3.18% — a diameter difference of 0.78″.
What changes between 215/45R17 and 235/40R18
Switching from 215/45R17 to 235/40R18 changes overall diameter by +3.18%, a taller tire by 0.78 inches. This sits in the 3–5% caution band, where rubbing at full steering lock or suspension compression becomes possible. Leveling kits or fender-liner trims are often needed on lifted trucks. Section width moves by +0.79″ (215mm → 235mm), which affects contact patch and fender-well clearance. Sidewall height shifts by -0.11″: a shorter sidewall sharpens steering response at the cost of ride comfort and rim protection. Revs per mile moves by -25, which translates directly to speedometer error — at an indicated 65 mph, actual speed becomes 67.1 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 | 215/45R17 | 235/40R18 |
|---|---|---|
| Overall diameter | 24.62″ | 25.4″ |
| Section width | 8.46″ (215 mm) | 9.25″ (235 mm) |
| Sidewall height | 3.81″ (45%) | 3.7″ (40%) |
| Rim diameter | 17″ | 18″ |
| Circumference | 77.34″ | 79.8″ |
| Revs per mile | 819 | 794 |
Interactive comparison
275/65R18 or "33" for inch aliases215/45R17
- Diameter
- 24.62″ / 625.3mm
- Section Width
- 8.46″
- Sidewall
- 3.81″ / 96.75mm
- Circumference
- 77.34″
- Revs/Mile
- 819
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 requiredAt an indicated 60 mph, your actual speed is 61.91 mph (99.63 km/h). Your speedometer reads 3.18% low versus actual speed.