Comparison
245/35 R20 vs 255/40 R18
255/40 R18 drops the rim from 20 to 18 inches versus 245/35 R20, trading wheel size for taller sidewall. This swap moves rolling diameter a touch off the original spec. More sidewall typically improves comfort and curb protection, especially on city streets.
Expect a slight but noticeable shift in indicated speed compared to the original tires. 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: 255/40 R18 is 18.3 mm shorter than 245/35 R20, shifting the speedometer by -2.69%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-18.3 mm
-2.69%
Speedometer at 100
97.3 km/h
-2.69% error
Ground clearance
-9.1 mm
ride height delta
Sidewall change
+16.3 mm
revs/km: 481.4
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/compare/245-35-r20-vs-255-40-r18Detailed comparison
| Metric | 245/35 R20 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 679.5 mm | 661.2 mm | -18.3 mm (-2.69%) |
| Sidewall height | 85.8 mm | 102.0 mm | +16.3 mm |
| Circumference | 2.135 m | 2.077 m | -57.5 mm |
| Revs / km | 468.4 | 481.4 | +13.0 |
| Ground clearance | reference | -9.1 mm | -9.1 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 97.3 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 97.3 km/h after switching to 255/40 R18 — 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 -9.1 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
245/35 R20
Back to
255/40 R18
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