Comparison
245/35 R18 vs 255/40 R18
255/40 R18 is a wider variation of 245/35 R18 on the same 18-inch rim, adding 10 mm of tread footprint. This wheel and tire pairing moves rolling diameter well outside the usual OEM tolerance.
The dashboard speed will be significantly off — plan on recalibration before daily use. More sidewall typically improves comfort and curb protection, especially on city streets. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. A diameter change beyond 5% is aggressive enough to influence ABS, traction control and gearing; treat it as a serious modification.
TakeTreat as a serious modification — verify clearance, recalibrate the speedometer and reassess load capacity.
Quick math: 255/40 R18 is 32.5 mm taller than 245/35 R18, shifting the speedometer by +5.17%.
Current Tire
New Tire
Ground line · scaled comparison
Not Recommended
Over 5% — speedometer & ABS may misread
Diameter change
+32.5 mm
5.17%
Speedometer at 100
105.2 km/h
+5.17% error
Ground clearance
+16.3 mm
ride height delta
Sidewall change
+16.3 mm
revs/km: 481.4
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/compare/245-35-r18-vs-255-40-r18Detailed comparison
| Metric | 245/35 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 628.7 mm | 661.2 mm | +32.5 mm (+5.17%) |
| Sidewall height | 85.8 mm | 102.0 mm | +16.3 mm |
| Circumference | 1.975 m | 2.077 m | +102.1 mm |
| Revs / km | 506.3 | 481.4 | -24.9 |
| Ground clearance | reference | +16.3 mm | +16.3 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 105.2 km/h | +5.17 km/h |
Verdict: danger
Over 5% diameter difference — likely to affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.
Speedometer impact
At a true 100 km/h, your dashboard will read 105.2 km/h after switching to 255/40 R18 — a +5.17% 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 +16.3 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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245/35 R18
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255/40 R18
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