Comparison
235/40 R18 vs 245/40 R17
245/40 R17 drops the rim from 18 to 17 inches versus 235/40 R18, trading wheel size for taller sidewall. This alternative fitment moves rolling diameter a touch off the original spec. The speedometer offset is small but measurable; worth keeping in mind if you watch the dash closely. More tread on the ground tends to improve dry grip and stance, with a small fuel-economy and clearance tradeoff. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 245/40 R17 is 17.4 mm shorter than 235/40 R18, shifting the speedometer by -2.70%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-17.4 mm
-2.70%
Speedometer at 100
97.3 km/h
-2.70% error
Ground clearance
-8.7 mm
ride height delta
Sidewall change
+4.0 mm
revs/km: 507.0
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/compare/235-40-r18-vs-245-40-r17Detailed comparison
| Metric | 235/40 R18 | 245/40 R17 | Difference |
|---|---|---|---|
| Overall diameter | 645.2 mm | 627.8 mm | -17.4 mm (-2.70%) |
| Sidewall height | 94.0 mm | 98.0 mm | +4.0 mm |
| Circumference | 2.027 m | 1.972 m | -54.7 mm |
| Revs / km | 493.4 | 507.0 | +13.7 |
| Ground clearance | reference | -8.7 mm | -8.7 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 97.3 km/h | -2.70 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 245/40 R17 — a -2.70% 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 -8.7 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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235/40 R18
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245/40 R17
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