Comparison
255/35 R21 vs 305/30 R20
Switching from 255/35 R21 to 305/30 R20 steps down to a 20-inch wheel — a familiar move for winter and dedicated all-terrain sets. This swap shifts overall diameter slightly from OEM. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement.
The speedometer offset is small but measurable; worth keeping in mind if you watch the dash closely. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeTypical choice for a dedicated winter or off-road setup where extra sidewall pays off.
Quick math: 305/30 R20 is 20.9 mm shorter than 255/35 R21, shifting the speedometer by -2.94%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-20.9 mm
-2.94%
Speedometer at 100
97.1 km/h
-2.94% error
Ground clearance
-10.4 mm
ride height delta
Sidewall change
+2.3 mm
revs/km: 460.7
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/compare/255-35-r21-vs-305-30-r20Detailed comparison
| Metric | 255/35 R21 | 305/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 711.9 mm | 691.0 mm | -20.9 mm (-2.94%) |
| Sidewall height | 89.3 mm | 91.5 mm | +2.3 mm |
| Circumference | 2.236 m | 2.171 m | -65.7 mm |
| Revs / km | 447.1 | 460.7 | +13.5 |
| Ground clearance | reference | -10.4 mm | -10.4 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 97.1 km/h | -2.94 km/h |
Verdict: excellent
Within ±3% — speedometer, ABS and traction control should behave normally.
Speedometer impact
At a true 100 km/h, your dashboard will read 97.1 km/h after switching to 305/30 R20 — a -2.94% 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 -10.4 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.
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255/35 R21
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305/30 R20
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