Question 1

Which tire is taller, 305/30 R20 or 285/35 R21?

Accepted Answer

285/35 R21 is taller by 41.9 mm (6.06%) compared with 305/30 R20.

Question 2

What is the diameter difference between 305/30 R20 and 285/35 R21?

Accepted Answer

305/30 R20 measures 691 mm overall diameter, 285/35 R21 measures 733 mm — a difference of 41.9 mm (6.06%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 305/30 R20 to 285/35 R21 changes the speedometer reading by +6.06%. At an indicated 100 km/h, your true speed becomes about 106.1 km/h.

Question 4

Is 285/35 R21 a safe replacement for 305/30 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	305/30 R20	285/35 R21	Difference
Overall diameter	691.0 mm	732.9 mm	+41.9 mm (+6.06%)
Sidewall height	91.5 mm	99.8 mm	+8.3 mm
Circumference	2.171 m	2.302 m	+131.6 mm
Revs / km	460.7	434.3	-26.3
Ground clearance	reference	+20.9 mm	+20.9 mm
Speedometer @ 100 km/h	100.0 km/h	106.1 km/h	+6.06 km/h

305/30 R20 vs 285/35 R21

Detailed comparison

Verdict: danger

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