Comparison
275/40 R18 vs 305/30 R20
Plus-sizing from 275/40 R18 to 305/30 R20 keeps overall diameter close to factory while opening room for a larger 20-inch wheel. This alternative fitment moves rolling diameter a touch off the original spec. Expect a slight but noticeable shift in indicated speed compared to the original tires. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeCommon upgrade for sportier handling and a tighter wheel-gap look on the same vehicle.
Quick math: 305/30 R20 is 13.8 mm taller than 275/40 R18, shifting the speedometer by +2.04%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+13.8 mm
2.04%
Speedometer at 100
102.0 km/h
+2.04% error
Ground clearance
+6.9 mm
ride height delta
Sidewall change
-18.5 mm
revs/km: 460.7
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/compare/275-40-r18-vs-305-30-r20Detailed comparison
| Metric | 275/40 R18 | 305/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 677.2 mm | 691.0 mm | +13.8 mm (+2.04%) |
| Sidewall height | 110.0 mm | 91.5 mm | -18.5 mm |
| Circumference | 2.127 m | 2.171 m | +43.4 mm |
| Revs / km | 470.0 | 460.7 | -9.4 |
| Ground clearance | reference | +6.9 mm | +6.9 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 102.0 km/h | +2.04 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 102.0 km/h after switching to 305/30 R20 — a +2.04% 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 +6.9 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.
Back to
275/40 R18
Back to
305/30 R20
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