Comparison
255/40 R18 vs 285/30 R20
Going from 255/40 R18 to 285/30 R20 steps up to a 20-inch rim while trimming sidewall to stay near OEM rolling diameter. This sizing approach trims or stretches rolling diameter by a small margin. The speedometer offset is small but measurable; worth keeping in mind if you watch the dash closely. The shorter sidewall gives the tire a firmer, more responsive feel and sharpens steering input. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 285/30 R20 is 17.8 mm taller than 255/40 R18, shifting the speedometer by +2.69%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+17.8 mm
2.69%
Speedometer at 100
102.7 km/h
+2.69% error
Ground clearance
+8.9 mm
ride height delta
Sidewall change
-16.5 mm
revs/km: 468.8
Permalink for this comparison:
/compare/255-40-r18-vs-285-30-r20Detailed comparison
| Metric | 255/40 R18 | 285/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 661.2 mm | 679.0 mm | +17.8 mm (+2.69%) |
| Sidewall height | 102.0 mm | 85.5 mm | -16.5 mm |
| Circumference | 2.077 m | 2.133 m | +55.9 mm |
| Revs / km | 481.4 | 468.8 | -12.6 |
| Ground clearance | reference | +8.9 mm | +8.9 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 102.7 km/h | +2.69 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 102.7 km/h after switching to 285/30 R20 — a +2.69% 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 +8.9 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
255/40 R18
Back to
285/30 R20
Drivers also compare
Closely-related fitments and plus-size swaps for 255/40 R18 and 285/30 R20.
245/35 R20 vs 285/30 R20
Wider variation on the same rim — more grip, less clearance.
Δ 0.07%
225/45 R18 vs 255/40 R18
Wider variation on the same rim — more grip, less clearance.
Δ 0.23%
255/35 R20 vs 285/30 R20
Wider variation on the same rim — more grip, less clearance.
Δ 1.09%
235/45 R18 vs 255/40 R18
Wider variation on the same rim — more grip, less clearance.
Δ 1.12%
255/40 R18 vs 265/40 R18
Wider variation on the same rim — more grip, less clearance.
Δ 1.21%
245/40 R18 vs 255/40 R18
Wider variation on the same rim — more grip, less clearance.
Δ 1.22%
215/45 R18 vs 255/40 R18
Wider variation on the same rim — more grip, less clearance.
Δ 1.61%
255/40 R18 vs 275/35 R18
Wider variation on the same rim — more grip, less clearance.
Δ 1.74%
Related topics