Question 1

Which tire is taller, 255/35 R21 or 315/30 R22?

Accepted Answer

315/30 R22 is taller by 35.9 mm (5.04%) compared with 255/35 R21.

Question 2

What is the diameter difference between 255/35 R21 and 315/30 R22?

Accepted Answer

255/35 R21 measures 712 mm overall diameter, 315/30 R22 measures 748 mm — a difference of 35.9 mm (5.04%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/35 R21 to 315/30 R22 changes the speedometer reading by +5.04%. At an indicated 100 km/h, your true speed becomes about 105.0 km/h.

Question 4

Is 315/30 R22 a safe replacement for 255/35 R21?

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	255/35 R21	315/30 R22	Difference
Overall diameter	711.9 mm	747.8 mm	+35.9 mm (+5.04%)
Sidewall height	89.3 mm	94.5 mm	+5.3 mm
Circumference	2.236 m	2.349 m	+112.8 mm
Revs / km	447.1	425.7	-21.5
Ground clearance	reference	+17.9 mm	+17.9 mm
Speedometer @ 100 km/h	100.0 km/h	105.0 km/h	+5.04 km/h

255/35 R21 vs 315/30 R22

Detailed comparison

Verdict: danger

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