Question 1

Which tire is taller, 255/35 R21 or 225/45 R17?

Accepted Answer

255/35 R21 is taller by 77.6 mm (10.90%) compared with 225/45 R17.

Question 2

What is the diameter difference between 255/35 R21 and 225/45 R17?

Accepted Answer

255/35 R21 measures 712 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 77.6 mm (10.90%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 255/35 R21 to 225/45 R17 changes the speedometer reading by -10.90%. At an indicated 100 km/h, your true speed becomes about 89.1 km/h.

Question 4

Is 225/45 R17 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	225/45 R17	Difference
Overall diameter	711.9 mm	634.3 mm	-77.6 mm (-10.90%)
Sidewall height	89.3 mm	101.3 mm	+12.0 mm
Circumference	2.236 m	1.993 m	-243.8 mm
Revs / km	447.1	501.8	+54.7
Ground clearance	reference	-38.8 mm	-38.8 mm
Speedometer @ 100 km/h	100.0 km/h	89.1 km/h	-10.90 km/h

255/35 R21 vs 225/45 R17

Detailed comparison

Verdict: danger

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