Question 1

Which tire is taller, 285/35 R21 or 255/55 R18?

Accepted Answer

255/55 R18 is taller by 4.8 mm (0.65%) compared with 285/35 R21.

Question 2

What is the diameter difference between 285/35 R21 and 255/55 R18?

Accepted Answer

285/35 R21 measures 733 mm overall diameter, 255/55 R18 measures 738 mm — a difference of 4.8 mm (0.65%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 285/35 R21 to 255/55 R18 changes the speedometer reading by +0.65%. At an indicated 100 km/h, your true speed becomes about 100.7 km/h.

Question 4

Is 255/55 R18 a safe replacement for 285/35 R21?

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	285/35 R21	255/55 R18	Difference
Overall diameter	732.9 mm	737.7 mm	+4.8 mm (+0.65%)
Sidewall height	99.8 mm	140.3 mm	+40.5 mm
Circumference	2.302 m	2.318 m	+15.1 mm
Revs / km	434.3	431.5	-2.8
Ground clearance	reference	+2.4 mm	+2.4 mm
Speedometer @ 100 km/h	100.0 km/h	100.7 km/h	+0.65 km/h

285/35 R21 vs 255/55 R18

Detailed comparison

Verdict: excellent

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