Question 1

Which tire is taller, 205/40 R17 or 255/35 R18?

Accepted Answer

255/35 R18 is taller by 39.9 mm (6.70%) compared with 205/40 R17.

Question 2

What is the diameter difference between 205/40 R17 and 255/35 R18?

Accepted Answer

205/40 R17 measures 596 mm overall diameter, 255/35 R18 measures 636 mm — a difference of 39.9 mm (6.70%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 205/40 R17 to 255/35 R18 changes the speedometer reading by +6.70%. At an indicated 100 km/h, your true speed becomes about 106.7 km/h.

Question 4

Is 255/35 R18 a safe replacement for 205/40 R17?

Accepted Answer

Over 5% diameter difference. This is outside the common safe range and can affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.

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	205/40 R17	255/35 R18	Difference
Overall diameter	595.8 mm	635.7 mm	+39.9 mm (+6.70%)
Sidewall height	82.0 mm	89.3 mm	+7.3 mm
Circumference	1.872 m	1.997 m	+125.3 mm
Revs / km	534.3	500.7	-33.5
Ground clearance	reference	+20.0 mm	+20.0 mm
Speedometer @ 100 km/h	100.0 km/h	106.7 km/h	+6.70 km/h

205/40 R17 vs 255/35 R18

Detailed comparison

Verdict: danger

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