235/45R17 vs 235/40R18
Side-by-side comparison of overall diameter, sidewall height, speedometer error, and ride-height change.
Safe to swap — Diameter difference is under 1.5% — within normal tolerance.
To scale
235/45R17
- Section width
- 235 mm
- Aspect ratio
- 45%
- Rim diameter
- 17"
- Sidewall
- 105.8 mm
- Overall diameter
- 643.3 mm
- Circumference
- 2021 mm
- Revs / mile
- 796
- Revs / km
- 495
235/40R18
- Section width
- 235 mm
- Aspect ratio
- 40%
- Rim diameter
- 18"
- Sidewall
- 94.0 mm
- Overall diameter
- 645.2 mm
- Circumference
- 2027 mm
- Revs / mile
- 794
- Revs / km
- 493
Difference
- Overall diameter
- +1.9 mm (+0.30%)
- Sidewall height
- -11.8 mm
- Tread width
- +0 mm
- Ride height (axle)
- +1.0 mm
- Speedometer error
- +0.30%
- At indicated 60 mph
- actual 60.2 mph
- At indicated 100 km/h
- actual 100.3 km/h
Will 235/40R18 fit instead of 235/45R17?
The overall diameter changes by 0.30%. Most manufacturers consider a swap acceptable when the difference stays under 3% — beyond that, you may see speedometer error, ABS/TPMS warnings, or wheel-well clearance problems. This page only covers the math; your vehicle's wheel arches, suspension geometry, and load rating still matter.
What the numbers mean
- Overall diameter — outside-to-outside height of the tire. Affects ride height, gearing, and speedometer calibration.
- Sidewall height — rubber from rim edge to tread. Lower sidewall gives sharper steering response but a harsher ride.
- Speedometer error — how far off your indicated speed will be after the swap. Positive means the speedo under-reads (actual speed is higher than shown).