Comparison
245/40 R18 vs 245/45 R18
Going from 245/40 R18 to 245/45 R18 raises the aspect ratio by 5 points on the same 18-inch wheel. This wheel and tire pairing swings rolling diameter far enough to feel on the road. The taller sidewall adds cushioning over potholes and rougher roads, with a softer overall ride.
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.
TakeA comfort-leaning direction that's easier on wheels and suspension over rough surfaces.
Quick math: 245/45 R18 is 24.5 mm taller than 245/40 R18, shifting the speedometer by +3.75%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+24.5 mm
3.75%
Speedometer at 100
103.8 km/h
+3.75% error
Ground clearance
+12.3 mm
ride height delta
Sidewall change
+12.3 mm
revs/km: 469.7
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/compare/245-40-r18-vs-245-45-r18Detailed comparison
| Metric | 245/40 R18 | 245/45 R18 | Difference |
|---|---|---|---|
| Overall diameter | 653.2 mm | 677.7 mm | +24.5 mm (+3.75%) |
| Sidewall height | 98.0 mm | 110.3 mm | +12.3 mm |
| Circumference | 2.052 m | 2.129 m | +77.0 mm |
| Revs / km | 487.3 | 469.7 | -17.6 |
| Ground clearance | reference | +12.3 mm | +12.3 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 103.8 km/h | +3.75 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 103.8 km/h after switching to 245/45 R18 — a +3.75% 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 +12.3 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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245/40 R18
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245/45 R18
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