Question 1

Which tire is taller, 285/60 R18 or 255/50 R20?

Accepted Answer

285/60 R18 is taller by 36.2 mm (4.53%) compared with 255/50 R20.

Question 2

What is the diameter difference between 285/60 R18 and 255/50 R20?

Accepted Answer

285/60 R18 measures 799 mm overall diameter, 255/50 R20 measures 763 mm — a difference of 36.2 mm (4.53%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 285/60 R18 to 255/50 R20 changes the speedometer reading by -4.53%. At an indicated 100 km/h, your true speed becomes about 95.5 km/h.

Question 4

Is 255/50 R20 a safe replacement for 285/60 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	285/60 R18	255/50 R20	Difference
Overall diameter	799.2 mm	763.0 mm	-36.2 mm (-4.53%)
Sidewall height	171.0 mm	127.5 mm	-43.5 mm
Circumference	2.511 m	2.397 m	-113.7 mm
Revs / km	398.3	417.2	+18.9
Ground clearance	reference	-18.1 mm	-18.1 mm
Speedometer @ 100 km/h	100.0 km/h	95.5 km/h	-4.53 km/h

285/60 R18 vs 255/50 R20

Detailed comparison

Verdict: warning

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