Question 1

Which tire is taller, 265/40 R21 or 225/45 R17?

Accepted Answer

265/40 R21 is taller by 111.1 mm (14.90%) compared with 225/45 R17.

Question 2

What is the diameter difference between 265/40 R21 and 225/45 R17?

Accepted Answer

265/40 R21 measures 745 mm overall diameter, 225/45 R17 measures 634 mm — a difference of 111.1 mm (14.90%).

Question 3

How much speedometer error does this change create?

Accepted Answer

Switching from 265/40 R21 to 225/45 R17 changes the speedometer reading by -14.90%. At an indicated 100 km/h, your true speed becomes about 85.1 km/h.

Question 4

Is 225/45 R17 a safe replacement for 265/40 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	265/40 R21	225/45 R17	Difference
Overall diameter	745.4 mm	634.3 mm	-111.1 mm (-14.90%)
Sidewall height	106.0 mm	101.3 mm	-4.8 mm
Circumference	2.342 m	1.993 m	-349.0 mm
Revs / km	427.0	501.8	+74.8
Ground clearance	reference	-55.6 mm	-55.6 mm
Speedometer @ 100 km/h	100.0 km/h	85.1 km/h	-14.90 km/h

265/40 R21 vs 225/45 R17

Detailed comparison

Verdict: danger

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