185/40R21 vs 185/45R21
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
185/40R21
- Section width
- 185 mm
- Aspect ratio
- 40%
- Rim
- 21"
- Sidewall
- 74.0 mm
- Overall diameter
- 681.4 mm
- Revs / mile
- 752
185/45R21
- Section width
- 185 mm
- Aspect ratio
- 45%
- Rim
- 21"
- Sidewall
- 83.3 mm
- Overall diameter
- 699.9 mm
- Revs / mile
- 732
Difference
- Overall diameter
- +18.5 mm (+2.71%)
- Sidewall
- +9.3 mm
- Tread width
- +0 mm
- Ride height (axle)
- +9.3 mm
- Speedometer error
- +2.71%
- At indicated 60 mph
- actual 61.63 mph
Will 185/45R21 fit instead of 185/40R21?
The overall diameter changes by 2.71%. 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).