Question 1

Which tire is taller, 245/45 R17 or 215/40 R18?

Accepted Answer

245/45 R17 is taller by 23.1 mm (3.54%) compared with 215/40 R18.

Question 2

What is the diameter difference between 245/45 R17 and 215/40 R18?

Accepted Answer

245/45 R17 measures 652 mm overall diameter, 215/40 R18 measures 629 mm — a difference of 23.1 mm (3.54%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 245/45 R17 to 215/40 R18 changes the speedometer reading by -3.54%. At an indicated 100 km/h, your true speed becomes about 96.5 km/h.

Question 4

Is 215/40 R18 a safe replacement for 245/45 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	245/45 R17	215/40 R18	Difference
Overall diameter	652.3 mm	629.2 mm	-23.1 mm (-3.54%)
Sidewall height	110.3 mm	86.0 mm	-24.3 mm
Circumference	2.049 m	1.977 m	-72.6 mm
Revs / km	488.0	505.9	+17.9
Ground clearance	reference	-11.5 mm	-11.5 mm
Speedometer @ 100 km/h	100.0 km/h	96.5 km/h	-3.54 km/h

245/45 R17 vs 215/40 R18

Detailed comparison

Verdict: warning

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