Question 1

Which tire is taller, 255/40 R18 or 225/45 R17?

Accepted Answer

255/40 R18 is taller by 26.9 mm (4.07%) compared with 225/45 R17.

Question 2

What is the diameter difference between 255/40 R18 and 225/45 R17?

Accepted Answer

255/40 R18 measures 661 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 26.9 mm (4.07%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/40 R18 to 225/45 R17 changes the speedometer reading by -4.07%. At an indicated 100 km/h, your true speed becomes about 95.9 km/h.

Question 4

Is 225/45 R17 a safe replacement for 255/40 R18?

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	255/40 R18	225/45 R17	Difference
Overall diameter	661.2 mm	634.3 mm	-26.9 mm (-4.07%)
Sidewall height	102.0 mm	101.3 mm	-0.8 mm
Circumference	2.077 m	1.993 m	-84.5 mm
Revs / km	481.4	501.8	+20.4
Ground clearance	reference	-13.5 mm	-13.5 mm
Speedometer @ 100 km/h	100.0 km/h	95.9 km/h	-4.07 km/h

255/40 R18 vs 225/45 R17

Detailed comparison

Verdict: warning

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