Question 1

Which tire is taller, 255/40 R17 or 235/40 R18?

Accepted Answer

235/40 R18 is taller by 9.4 mm (1.48%) compared with 255/40 R17.

Question 2

What is the diameter difference between 255/40 R17 and 235/40 R18?

Accepted Answer

255/40 R17 measures 636 mm overall diameter, 235/40 R18 measures 645 mm — a difference of 9.4 mm (1.48%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/40 R17 to 235/40 R18 changes the speedometer reading by +1.48%. At an indicated 100 km/h, your true speed becomes about 101.5 km/h.

Question 4

Is 235/40 R18 a safe replacement for 255/40 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	255/40 R17	235/40 R18	Difference
Overall diameter	635.8 mm	645.2 mm	+9.4 mm (+1.48%)
Sidewall height	102.0 mm	94.0 mm	-8.0 mm
Circumference	1.997 m	2.027 m	+29.5 mm
Revs / km	500.6	493.4	-7.3
Ground clearance	reference	+4.7 mm	+4.7 mm
Speedometer @ 100 km/h	100.0 km/h	101.5 km/h	+1.48 km/h

255/40 R17 vs 235/40 R18

Detailed comparison

Verdict: excellent

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