165/40R21 vs 175/40R21
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
165/40R21
- Section width
- 165 mm
- Aspect ratio
- 40%
- Rim
- 21"
- Sidewall
- 66.0 mm
- Overall diameter
- 665.4 mm
- Revs / mile
- 770
175/40R21
- Section width
- 175 mm
- Aspect ratio
- 40%
- Rim
- 21"
- Sidewall
- 70.0 mm
- Overall diameter
- 673.4 mm
- Revs / mile
- 761
Difference
- Overall diameter
- +8.0 mm (+1.20%)
- Sidewall
- +4.0 mm
- Tread width
- +10 mm
- Ride height (axle)
- +4.0 mm
- Speedometer error
- +1.20%
- At indicated 60 mph
- actual 60.72 mph
Will 175/40R21 fit instead of 165/40R21?
The overall diameter changes by 1.20%. 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).