Question 1

Which tire is taller, 235/40 R18 or 235/45 R17?

Accepted Answer

235/40 R18 is taller by 1.9 mm (0.29%) compared with 235/45 R17.

Question 2

What is the diameter difference between 235/40 R18 and 235/45 R17?

Accepted Answer

235/40 R18 measures 645 mm overall diameter, 235/45 R17 measures 643 mm — a difference of 1.9 mm (0.29%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 235/40 R18 to 235/45 R17 changes the speedometer reading by -0.29%. At an indicated 100 km/h, your true speed becomes about 99.7 km/h.

Question 4

Is 235/45 R17 a safe replacement for 235/40 R18?

Accepted Answer

Within ±3% diameter — generally considered a safe replacement from a speedometer-error standpoint, provided wheel offset, rim width and clearance also match your vehicle.

Question 5

What else should I check before changing tire size?

Accepted Answer

Beyond diameter and speedometer error, verify wheel offset (ET), rim width, hub bore, load index, brake and suspension clearance, and your vehicle's manufacturer specification. Local regulations may also limit how far you can deviate from OEM.

Metric	235/40 R18	235/45 R17	Difference
Overall diameter	645.2 mm	643.3 mm	-1.9 mm (-0.29%)
Sidewall height	94.0 mm	105.8 mm	+11.8 mm
Circumference	2.027 m	2.021 m	-6.0 mm
Revs / km	493.4	494.8	+1.5
Ground clearance	reference	-1.0 mm	-1.0 mm
Speedometer @ 100 km/h	100.0 km/h	99.7 km/h	-0.29 km/h

235/40 R18 vs 235/45 R17

Detailed comparison

Verdict: excellent

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