Comparison
275/35 R21 vs 305/30 R20
Going from 275/35 R21 to 305/30 R20 is a minus-1 setup that adds sidewall on a smaller 20-inch wheel. This tire combination moves rolling diameter well outside the usual OEM tolerance.
The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement. Extra width broadens the footprint for more grip, but check inner liner and strut clearance before fitting. Minus-sizing keeps replacement costs down and opens up a wider range of winter and all-terrain tires. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 305/30 R20 is 34.9 mm shorter than 275/35 R21, shifting the speedometer by -4.81%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-34.9 mm
-4.81%
Speedometer at 100
95.2 km/h
-4.81% error
Ground clearance
-17.4 mm
ride height delta
Sidewall change
-4.8 mm
revs/km: 460.7
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/compare/275-35-r21-vs-305-30-r20Detailed comparison
| Metric | 275/35 R21 | 305/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 725.9 mm | 691.0 mm | -34.9 mm (-4.81%) |
| Sidewall height | 96.3 mm | 91.5 mm | -4.8 mm |
| Circumference | 2.280 m | 2.171 m | -109.6 mm |
| Revs / km | 438.5 | 460.7 | +22.1 |
| Ground clearance | reference | -17.4 mm | -17.4 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 95.2 km/h | -4.81 km/h |
Verdict: warning
Between 3% and 5% — noticeable speedometer drift; recalibration may be advisable.
Speedometer impact
At a true 100 km/h, your dashboard will read 95.2 km/h after switching to 305/30 R20 — a -4.81% offset. Use the speedometer error calculator to recheck for other indicated speeds, and the speedometer error guide for the full background.
Ground clearance change
The new tire's half-diameter changes ride height by -17.4 mm. Small differences are absorbed by suspension travel, but anything beyond ±10 mm can affect headlight aim, fender clearance and bump-stop margin. See the plus-sizing guide before committing.
Back to
275/35 R21
Back to
305/30 R20
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