Comparison
265/40 R18 vs 305/30 R20
305/30 R20 is a plus-2 alternative to 265/40 R18 — the bigger wheel shows through a thinner sidewall. This wheel and tire pairing noticeably changes overall diameter compared to OEM. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides.
Indicated speed will drift far enough that recalibration is worth considering. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakeCommon upgrade for sportier handling and a tighter wheel-gap look on the same vehicle.
Quick math: 305/30 R20 is 21.8 mm taller than 265/40 R18, shifting the speedometer by +3.26%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+21.8 mm
3.26%
Speedometer at 100
103.3 km/h
+3.26% error
Ground clearance
+10.9 mm
ride height delta
Sidewall change
-14.5 mm
revs/km: 460.7
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/compare/265-40-r18-vs-305-30-r20Detailed comparison
| Metric | 265/40 R18 | 305/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 669.2 mm | 691.0 mm | +21.8 mm (+3.26%) |
| Sidewall height | 106.0 mm | 91.5 mm | -14.5 mm |
| Circumference | 2.102 m | 2.171 m | +68.5 mm |
| Revs / km | 475.7 | 460.7 | -15.0 |
| Ground clearance | reference | +10.9 mm | +10.9 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 103.3 km/h | +3.26 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 103.3 km/h after switching to 305/30 R20 — a +3.26% 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 +10.9 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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265/40 R18
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305/30 R20
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