275/45R20 vs 275/40R21
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
275/45R20
- Section width
- 275 mm
- Aspect ratio
- 45%
- Rim
- 20"
- Sidewall
- 123.8 mm
- Overall diameter
- 755.5 mm
- Revs / mile
- 678
275/40R21
- Section width
- 275 mm
- Aspect ratio
- 40%
- Rim
- 21"
- Sidewall
- 110.0 mm
- Overall diameter
- 753.4 mm
- Revs / mile
- 680
Difference
- Overall diameter
- -2.1 mm (-0.28%)
- Sidewall
- -13.8 mm
- Tread width
- +0 mm
- Ride height (axle)
- -1.1 mm
- Speedometer error
- -0.28%
- At indicated 60 mph
- actual 59.83 mph
Will 275/40R21 fit instead of 275/45R20?
The overall diameter changes by 0.28%. 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).