Question 1

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

Accepted Answer

225/45 R18 is taller by 23.9 mm (3.76%) compared with 255/40 R17.

Question 2

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

Accepted Answer

255/40 R17 measures 636 mm overall diameter, 225/45 R18 measures 660 mm — a difference of 23.9 mm (3.76%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/40 R17 to 225/45 R18 changes the speedometer reading by +3.76%. At an indicated 100 km/h, your true speed becomes about 103.8 km/h.

Question 4

Is 225/45 R18 a safe replacement for 255/40 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	255/40 R17	225/45 R18	Difference
Overall diameter	635.8 mm	659.7 mm	+23.9 mm (+3.76%)
Sidewall height	102.0 mm	101.3 mm	-0.8 mm
Circumference	1.997 m	2.073 m	+75.1 mm
Revs / km	500.6	482.5	-18.1
Ground clearance	reference	+12.0 mm	+12.0 mm
Speedometer @ 100 km/h	100.0 km/h	103.8 km/h	+3.76 km/h

255/40 R17 vs 225/45 R18

Detailed comparison

Verdict: warning

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