Question 1

Which tire is taller, 215/50 R17 or 215/40 R18?

Accepted Answer

215/50 R17 is taller by 17.6 mm (2.72%) compared with 215/40 R18.

Question 2

What is the diameter difference between 215/50 R17 and 215/40 R18?

Accepted Answer

215/50 R17 measures 647 mm overall diameter, 215/40 R18 measures 629 mm — a difference of 17.6 mm (2.72%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 215/50 R17 to 215/40 R18 changes the speedometer reading by -2.72%. At an indicated 100 km/h, your true speed becomes about 97.3 km/h.

Question 4

Is 215/40 R18 a safe replacement for 215/50 R17?

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	215/50 R17	215/40 R18	Difference
Overall diameter	646.8 mm	629.2 mm	-17.6 mm (-2.72%)
Sidewall height	107.5 mm	86.0 mm	-21.5 mm
Circumference	2.032 m	1.977 m	-55.3 mm
Revs / km	492.1	505.9	+13.8
Ground clearance	reference	-8.8 mm	-8.8 mm
Speedometer @ 100 km/h	100.0 km/h	97.3 km/h	-2.72 km/h

215/50 R17 vs 215/40 R18

Detailed comparison

Verdict: excellent

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