175/30R21 vs 185/30R21
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
175/30R21
- Section width
- 175 mm
- Aspect ratio
- 30%
- Rim
- 21"
- Sidewall
- 52.5 mm
- Overall diameter
- 638.4 mm
- Revs / mile
- 802
185/30R21
- Section width
- 185 mm
- Aspect ratio
- 30%
- Rim
- 21"
- Sidewall
- 55.5 mm
- Overall diameter
- 644.4 mm
- Revs / mile
- 795
Difference
- Overall diameter
- +6.0 mm (+0.94%)
- Sidewall
- +3.0 mm
- Tread width
- +10 mm
- Ride height (axle)
- +3.0 mm
- Speedometer error
- +0.94%
- At indicated 60 mph
- actual 60.56 mph
Will 185/30R21 fit instead of 175/30R21?
The overall diameter changes by 0.94%. 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).