Question 1

Which tire is taller, 235/55 R17 or 215/45 R18?

Accepted Answer

235/55 R17 is taller by 39.6 mm (5.74%) compared with 215/45 R18.

Question 2

What is the diameter difference between 235/55 R17 and 215/45 R18?

Accepted Answer

235/55 R17 measures 690 mm overall diameter, 215/45 R18 measures 651 mm — a difference of 39.6 mm (5.74%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 235/55 R17 to 215/45 R18 changes the speedometer reading by -5.74%. At an indicated 100 km/h, your true speed becomes about 94.3 km/h.

Question 4

Is 215/45 R18 a safe replacement for 235/55 R17?

Accepted Answer

Over 5% diameter difference. This is outside the common safe range and can affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.

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	235/55 R17	215/45 R18	Difference
Overall diameter	690.3 mm	650.7 mm	-39.6 mm (-5.74%)
Sidewall height	129.3 mm	96.8 mm	-32.5 mm
Circumference	2.169 m	2.044 m	-124.4 mm
Revs / km	461.1	489.2	+28.1
Ground clearance	reference	-19.8 mm	-19.8 mm
Speedometer @ 100 km/h	100.0 km/h	94.3 km/h	-5.74 km/h

235/55 R17 vs 215/45 R18

Detailed comparison

Verdict: danger

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