Question 1

Which tire is taller, 215/45 R17 or 235/40 R18?

Accepted Answer

235/40 R18 is taller by 19.9 mm (3.18%) compared with 215/45 R17.

Question 2

What is the diameter difference between 215/45 R17 and 235/40 R18?

Accepted Answer

215/45 R17 measures 625 mm overall diameter, 235/40 R18 measures 645 mm — a difference of 19.9 mm (3.18%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 215/45 R17 to 235/40 R18 changes the speedometer reading by +3.18%. At an indicated 100 km/h, your true speed becomes about 103.2 km/h.

Question 4

Is 235/40 R18 a safe replacement for 215/45 R17?

Accepted Answer

Between 3% and 5% diameter difference — speedometer drift is noticeable. Consider recalibration and confirm clearance with your vehicle's manufacturer specification.

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	215/45 R17	235/40 R18	Difference
Overall diameter	625.3 mm	645.2 mm	+19.9 mm (+3.18%)
Sidewall height	96.8 mm	94.0 mm	-2.8 mm
Circumference	1.964 m	2.027 m	+62.5 mm
Revs / km	509.1	493.4	-15.7
Ground clearance	reference	+10.0 mm	+10.0 mm
Speedometer @ 100 km/h	100.0 km/h	103.2 km/h	+3.18 km/h

215/45 R17 vs 235/40 R18

Detailed comparison

Verdict: warning

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