305/25R21 vs 305/30R21
Side-by-side comparison of overall diameter, sidewall height, speedometer error, and ride-height change.
Not recommended — Exceeds the 3% diameter limit — speedometer error and ABS/TPMS issues likely.
To scale
305/25R21
- Section width
- 305 mm
- Aspect ratio
- 25%
- Rim diameter
- 21"
- Sidewall
- 76.3 mm
- Overall diameter
- 685.9 mm
- Circumference
- 2155 mm
- Revs / mile
- 747
- Revs / km
- 464
305/30R21
- Section width
- 305 mm
- Aspect ratio
- 30%
- Rim diameter
- 21"
- Sidewall
- 91.5 mm
- Overall diameter
- 716.4 mm
- Circumference
- 2251 mm
- Revs / mile
- 715
- Revs / km
- 444
Difference
- Overall diameter
- +30.5 mm (+4.45%)
- Sidewall height
- +15.3 mm
- Tread width
- +0 mm
- Ride height (axle)
- +15.3 mm
- Speedometer error
- +4.45%
- At indicated 60 mph
- actual 62.7 mph
- At indicated 100 km/h
- actual 104.4 km/h
Will 305/30R21 fit instead of 305/25R21?
The overall diameter changes by 4.45%. 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 — outside-to-outside height of the tire. Affects ride height, gearing, and speedometer calibration.
- Sidewall height — rubber from rim edge to tread. Lower sidewall gives sharper steering response but a harsher ride.
- Speedometer error — how far off your indicated speed will be after the swap. Positive means the speedo under-reads (actual speed is higher than shown).