Question 1

Which tire is taller, 255/50 R20 or 255/35 R21?

Accepted Answer

255/50 R20 is taller by 51.1 mm (6.70%) compared with 255/35 R21.

Question 2

What is the diameter difference between 255/50 R20 and 255/35 R21?

Accepted Answer

255/50 R20 measures 763 mm overall diameter, 255/35 R21 measures 712 mm — a difference of 51.1 mm (6.70%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/50 R20 to 255/35 R21 changes the speedometer reading by -6.70%. At an indicated 100 km/h, your true speed becomes about 93.3 km/h.

Question 4

Is 255/35 R21 a safe replacement for 255/50 R20?

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	255/50 R20	255/35 R21	Difference
Overall diameter	763.0 mm	711.9 mm	-51.1 mm (-6.70%)
Sidewall height	127.5 mm	89.3 mm	-38.3 mm
Circumference	2.397 m	2.236 m	-160.5 mm
Revs / km	417.2	447.1	+29.9
Ground clearance	reference	-25.6 mm	-25.6 mm
Speedometer @ 100 km/h	100.0 km/h	93.3 km/h	-6.70 km/h

255/50 R20 vs 255/35 R21

Detailed comparison

Verdict: danger

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