175/40R18 vs 175/45R18
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
175/40R18
- Section width
- 175 mm
- Aspect ratio
- 40%
- Rim
- 18"
- Sidewall
- 70.0 mm
- Overall diameter
- 597.2 mm
- Revs / mile
- 858
175/45R18
- Section width
- 175 mm
- Aspect ratio
- 45%
- Rim
- 18"
- Sidewall
- 78.8 mm
- Overall diameter
- 614.7 mm
- Revs / mile
- 833
Difference
- Overall diameter
- +17.5 mm (+2.93%)
- Sidewall
- +8.8 mm
- Tread width
- +0 mm
- Ride height (axle)
- +8.8 mm
- Speedometer error
- +2.93%
- At indicated 60 mph
- actual 61.76 mph
Will 175/45R18 fit instead of 175/40R18?
The overall diameter changes by 2.93%. 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 is the outside-to-outside height of the tire. Affects ride height, gearing, and speedometer.
- Sidewall is the height of the rubber from rim to tread. Lower sidewall = sharper steering, harsher ride.
- Speedometer error is the percentage your indicated speed will be off after the swap (positive = speedo reads lower than actual).